# ’98 Frontier 4WD occasional stalling on idle and occasional sputtering



## 98Frontier4WD (Jan 12, 2015)

Have a ’98 4WD Frontier, 160K miles.

As of 4 days ago I’ve had occasional stalling on idle (perhaps 1 out of 10 times I stop at a stoplight) and also occasional very brief engine hesitations at cruising speeds, akin to a, say, brief “hiccup” in engine power (perhaps a dozen episodes in a 250 mile trip).

It first happened 200 miles away from home on my 4th of July trip. Not a good situation.

No engine light and no codes (I read them).

Concerned about being left stranded, I tried to debug the thing myself since everything was closed for 4th of July. Ended up suspecting the IACV (Idle Air Control Valve) as something that would likely result in this behavior and would also likely not generate a code. But I did not get anywhere. I just sprayed some carburetor cleaner in the throttle air bypass hole, which seemed to make things a little worse, as it seemed to be stalling more often – but it could have been the higher engine temperature too, after repeated starts. Taking apart to clean the IACV was out of the question since I was going to have to remove the throttle body and did not have spare seals to reassemble (seals would likely rip on disassembly after 17 years).

Concerned about the 200+ mile return trip, I took truck to only garage I found open. Very nice guy, pleasant to work with and seemed competent. After running diagnostics he said the MAF signal was noisy and that Nissan had a TSB about adding an additional ground wire to the MAF sensor. He put the extra ground but things did not improve. So he said it was the electrical connector to the MAF that was faulty (logical). He actually demonstrated to me how with the engine on idle, pressing on the MAF electrical connector caused the engine to stall. Not sure how many times he tried that and whether it might have been a coincidence. Unable to find a connector on Sunday, he tried to fix it by bending a bit the pins and when that did not work trying to insert small wires in the connector to achieve better contact, but that did not work either.

At this point I started suspecting that indeed the problem might be at the MAF but not with the connector. Rather with the MAF itself. I thought it was more logical that the pressure he was applying to the connector somehow transferred inside the MAF and that is where the problem is. Otherwise, I could not see how a marginal connector can be disassembled two, three, five times and always get that marginal connection. Typically futzing with a marginal connector at all makes it either establish contact or drop contact completely. And dropping contact completely would have caused a code (I had tested that myself, removing the MAF connector threw the expected P0100 code and set the computer in safe mode with intentional fuel cutoff above 2000RPM). 

In any case, he was not able to fix it, but I successfully made the 200+ mile trip back home, with just one stall on idle at stoplights and about ten “hiccups” at freeway speeds. Otherwise, engine behavior was, I would say, normal, or perhaps near normal overall.

BTW, the mechanic was very nice. I wanted to pay him at least something for the 1.5 hours he spent working on my truck, even if he did not ultimately fix it, but he insisted that he did not want any money since he was unable to fix it. So I left feeling kind of bad -- I thought I should at least buy him a gift next time I go by that area (which I tend to do once a month).

In any case, now that I’m back home I’d like to see if I can fix this problem, so before I do anything I wanted to ask,

Anyone else had similar issues or insights?

A couple of other things I noticed:

No engine light and no codes

Took the spark plugs out on my first diagnostic attempt and they were all 4 darker than usual, not wet but with more carbon than usual. Right before I took spark plugs out truck had been idling for a while and then had stalled by itself. I did not try to look at spark plugs after a normal drive. 

Even when the engine is not stalling on idle, the idle speed is sometimes a little erratic, but not always.

Malfunction may be more prominent in hot desert air than cooler moist air. But it’s a rather recent problem, so I’m not sure, could be coincidence. 

Starts fine and no problems while engine cold. Any intermittent problems only start after engine has warmed up. 

The stall is most repeatable when I rev up engine in neutral and then release the gas and let it go to idle. That is when it typically stalls. 

Sometimes as the idle RPM drops below normal, it revs up and recovers. 

Fast idle when cold works as expected- also higher RPM when AC on also works as expected

I moved around spark plug wires several times while engine was idling, no difference

I looked at the MAF sensor, looks clean

I intentionally disconnected MAF to see if problem would go away in computer safe mode. Did not completely stall but engine operation without MAF was quite erratic, rough idle and not able to go above 2200 rpm (I think this is expected since computer goes into safe mode). But erratic behavior makes it hard to come to conclusions. I did get the P0100 code disconnecting the MAF, so some error detection is working. 

I looked at the IACV (Idle Air Control Valve). I can hear it clicking when I disconnect and reconnect it while ignition on and engine off. Measured correct solenoid resistance ( 10.9ohms). Got the correct codes (P505) when I disconnected it for more than a couple of seconds.

I noticed that the IACV solenoid buzzes just a tad bit with ignition on and engine not running (when everything is quiet). But not a loud buzzing. 

My speedometer is sometimes acting funny, showing higher or lower speed than normal (+-20 mph), but this has been going on for quite a while (a year or so) and seems to be unrelated to this stalling problem. Definitely predates it by a long shot.

No other prior major problems. Though a 4 banger I love the truck, has served me well. I maintain it well – myself. Yes, I have had the typical O2 sensor going bad here and there and a couple of minor coolant leaks from 15+ year hoses, all expected. 

Thanks for reading. 
Any advice appreciated.


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## rogoman (Dec 16, 2004)

Several things to check:

- Replace the fuel filter and air filter.
- Check or replace spark plugs. Use only OEM NGK plugs; any other types can cause driveability problems.
- Check the fuel pressure. Tee-in a tempoarary fuel pressure gauge at the output side of the fuel filter. The readings at idle should be as follows:
* with vacuum hose connected to the fuel pressure regulator: 34 psi
* with vacuum hose disconnected from the fuel pressure regulator: 43 psi
- Check for a major vacuum leak in the intake system. To check the intake system for a vacuum leak, attach a vacuum gauge to a full vacuum source. With the engine fully warmed up, the reading at idle should be 18 - 20 InHg. At 3,000 RPM, it should be 21 InHg. If readings are under 18 InHg, check the intake manifold nuts to make sure they are tight. The gasket may have failed; spray a water mist at the gasket to see if the gauge reading changes. Also check the intake plenum bellows at the throttle valve and at the MAF for cracks or loose clamps.
- The fuel injectors may be dirty. Run some good injection cleaner, like Techron or Redline SL-1, through the system; give the cleaner about a week or two to do it's job.


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## 98Frontier4WD (Jan 12, 2015)

I started engine without air filter and focused on the MAFS: 

CLEANED MAFS—NO EFFECT:
I cleaned the MAFS with MAFS cleaner (it was a bit dirty after all-had to look at it with magnifying lens) -- no effect. Still stalled at about 10% of stoplights and had more hesitation and sputtering than before. ~10 episodes in a 15 mile trip.

RECREATE STALL BY FUTZING WITH MAF—NO EFFECT:
I cannot recreate the condition the mechanic showed me whereby engine was stalling by pressing on the MAFS electrical contactor. I wiggled/ bent/ pressed on the contactor and its cables, no difference, just a not so healthy idle throughout. Perhaps what the mechanic showed me was just a coincidence at the time.

TRY HOT/COLD AIR ON MAF—NO EFFECT
I alternated blowing hot and cold air on the MAF housing using a blow drier, to test my previous temperature dependence theory. Again no difference. Note: because my MAFS is right on the throttle body I had to cut a piece of cardboard to shield the MAFS air hole from the blow dryer air flow, otherwise if the blow dryer airflow gets close to the MAFS air intake it disturbs the air flow enough to confuse the MAFS and then the engine does indeed stall.

STILL NO CODES.

Rogoman, thanks for the suggestions. Here is my speculative investigation priority at this point:

Spark plugs: I’ll look at them one more time to see if they still show “rich” condition after normal drive. They are NGK. The fact that they were all uniformly showing “rich” condition on my first inspection (see photos below) makes me want to give this low priority. Spark plug carbon is likely symptom not cause.
Air filter: Low priority. Filter is still clean, and engine malfunctions the same way even without the air filter.
Fuel filter: Low priority because there is no hesitation under high engine horsepower. Also seems like a clogged filter would be associated more with “lean” combustion, not the “rich” I observe.
Fuel pressure: High priority? High fuel pressure (regulator fail) seems plausible. Would likely cause “rich” combustion.
Pump bad less likely, again that would correlate to “lean”.
Vacuum leak: Low priority? Would’t this also result in “lean” ? 
Fuel injectors: Low priority. The fact that all 4 plugs seem uniformly sooted is not consistent. If I saw difference in one or two cylinders only then I would suspect injectors more. 

Here is a photo of how spark plugs looked three days ago (Don’t they look “rich”?):


And here is how they looked in 2009 (I regard these as normal looking, right?): 


More tests coming…


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## rogoman (Dec 16, 2004)

Yes, the plugs do show a rich condition in the first picture; second picture is what they should look like.

It's still possible that all the injectors are dirty and are slightly leaking causing an irregular spray pattern which might be causing incomplete combustion. It won't hurt to run some good injection cleaner.

You might also consider replacing the O2 sensor and the temperature sensor that the ECU uses; a bad temp sensor can definitely create a rich condition.


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## 98Frontier4WD (Jan 12, 2015)

Here is how spark plugs look today after 18 mile drive( 10mi freeway + 8mi city). About 10 short episodes of sputtering at highway speeds and two stalls at stoplights.

Do these still indicate “rich” combustion?


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## rogoman (Dec 16, 2004)

After 18 mi of driving, the plugs look normal; not rich at all.


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## 98Frontier4WD (Jan 12, 2015)

Thanks.
So perhaps I only have rich condition at idle and other instances where throttle is closed. I'll investigate the IACV then...


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## Roader (Nov 9, 2014)

> ...occasional stalling on idle (perhaps 1 out of 10 times I stop at a stoplight) and also occasional very brief engine hesitations at cruising speeds, akin to a, say, brief “hiccup” in engine power (perhaps a dozen episodes in a 250 mile trip)


An intermittant EGR problem could cause both problems. The FSM has diagnostic procedures for EGR and also what they call "non-detectable" (by the on-board diagnostics) problems.


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## 98Frontier4WD (Jan 12, 2015)

NOISY / UNRELIABLE MAF SIGNAL? 

I think I may actually have a noisy / unreliable MAF signal.

Look at this first video. 

With the engine NOT running and the ignition on, I taped the MAF air hole at the throttle body with a piece of electrical tape (the MAF in this 98 Frontier is in the throttle body). You can actually see the black strip of electrical tape at the very top of the video.

I inserted a paper clip into the MAF signal pin (the signal that tells the computer what the airflow is) to measure the signal.

In the beginning, the signal out of the MAF is a low 0.76V. This is a low value but I have zero airflow. Not sure what the signal should be with 0 airflow (but I measured the signal at idle and it is within spec). 

But now notice what happens when I press both the connector and the MAF housing. The voltage starts fluctuating, sometimes going as high as 1.30 V. I also show how striking the MAF housing makes the signal fluctuate. 

I did make sure I had a good connection to the wire through the paper clip and alligator clips, to make sure I was not just measuring bad connections at the measurement apparatus.

So is this normal? Is it normal for the signal to fluctuate like that with the engine not running? 

http://youtu.be/-xdD0uazA9Q


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## rogoman (Dec 16, 2004)

The small fluctuation in voltage may be due to an electrical capacitive affect between your body and the signal wires. The signal wires run through a grounded shield; make sure the shield is grounded by checking it with your multimeter.


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## 98Frontier4WD (Jan 12, 2015)

Thanks for the pointer rogoman,
But where is the shield? It must stop before it gets all the way to the connector. Should I strip some of the cable's outer plastic or tape to get to it?
I see in the wiring diagram that the shielding is grounded through a connector in the cabin, an enclosure by the passenger feet.


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## rogoman (Dec 16, 2004)

If the engine cable has additional wires besides the MAF signal wires, then it's probably safe to strip the cable's outer plastic or tape wrap without destroying the shield. If the cable has been tampered with, then it's possible that the shield may be damaged. If you're able to access the shield at the MAF harness connector, do a continuity test from the shield to engine ground to insure that it's grounded.


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## 98Frontier4WD (Jan 12, 2015)

MAF SIGNAL SHIELDING IS PROPERLY GROUNDED:

I tested the MAF signal cable shielding for proper ground. It seems to be well grounded.
I peeled back the cable wrapping, the shielding started about ½” from the MAF connector. 
Shielding is at 0V (well at 0.006V to be exact). Also to verify that I had a good ground I hooked a 7W light bulb ([email protected] 12V) to the battery (+) and grounded it through the shielding and it lit properly. So the ground looks pretty solid.


BUT SCOPE SHOWS NOISY MAF SIGNAL?

In the following video, I borrowed a potable scope to look at the MAF signal. The video shows the MAF signal while I press either on the MAF housing or the MAF connector. The engine is idling most of the time, though I do rev it up in a couple of instances. You can hear the uneven idling (yes I have a noisy belt pulley too).

Other than the electrical noise the MAF signal voltage looks within specs. Specs are 0.9-1.8V at idle and 1.9-2.3V at 2500rpm. As you can see in the video my idle voltage is about 1.6V at idle and 2.2V when engine is revved up.

This MAF signal looks noisy to me. I have never looked at any automotive signals with a scope before, so I don’t know what to expect. But still that much noise does not look normal to me. 

https://www.youtube.com/watch?v=0oaSzRDEAwQ


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## 98Frontier4WD (Jan 12, 2015)

1)	Five days ago, I did a crude test for VACUUM leaks:

I made some contraption with a glass jar and two hoses, lit an olive oil drenched paper towel in the jar, closed it and blew some smoke through the brake booster vacuum line. 

I did not see any vacuum leaks, though, admittedly I could not make a lot of smoke this way. But seems to me I should have discovered any major leaks even with this crude device.


2)	So (four days ago) I CHANGED THE MAF SENSOR:

Behavior changed, I no longer get the occasional hiccup at cruising speeds but problem with idle persists.

I did not want to buy an OEM MAFS just on suspicion (It is about $400) so I got a cheap one online for about $35, just to try it out. 

On the positive side, the new MAFS does not have a noisy signal like my original. When I wiggle and apply pressure on the MAFS electrical connector or the MAFS housing with the engine not running and ignition on, the signal I read with a voltmeter stays constant. Also the new MAFS seems to respond to the engine being revved up -- the voltage increases. I have not looked at the signal with a scope but I’m pretty sure its more stable. Is it more accurate than my old MAFS? Well that I don’t know. 

But in any case, with the new MAFS I don’t seem to get the short power hiccups I was experiencing before at cruising speed. I have now driven back and forth 3 times to work (6x20 miles) and no cruising speed hiccups. I used to get 4-5 hiccups per trip before.

However, the idle is still erratic when I go to work, but not when I return! Also, I never get an erratic idle immediately. Certainly not when the engine is warming up (fast idle). But even after the engine reaches operating temperature, still the idle is normal, in the beginning. Then, going to work, after I have driven a good 20’-25’ on the freeway and then continue on city traffic THEN I notice the rough idle at stoplights. Then the idle goes between 800 and 500 rpm and the engine feels like it’s going to stall, but never quite does. No codes! In a way, the rough idle is now worse with this new MAFS, but surprisingly, it never stalls like it used to with the previous MAFS.

Returning from work late in the evening, when the driving pattern is reverse, I get all the way home without any rough idling (idle is not perfect but I would say not pathological in this case). In this reverse driving pattern, I first drive 6 miles on city streets and stoplights and I don’t get any rough idle. The engine reaches operating temperature within the first 2 miles. Then I drive about 12 miles on freeway and then finally 2 miles on city streets and stoplights close to my house. I don’t get any rough idling on these last two miles. Basically I don’t get any rough idling at all on my return trip home. 

When I changed the MAFS, I read a pending code P0171 (mixture lean), after one round trip to work (2x20miles). But it was only a pending code, and I think it’s was likely because the new MAFS is different than the old one. I cleared the code and did not get another one in the next two days driving back and forth to work.

In any case, if it were a vacuum leak, it would not take 30’ to manifest itself. Would it? I imagine once the engine warmed up (5mins at most) it would have started causing symptoms. No?


3)	So (today) I TOOK APART AND CLEANED THE IACV (Idle Air Control Valve):

Was challenging to remove without removing the throttle body, but with some contortions and McGuyver screwdrivers I managed. 

?? Now, QUESTION: is the IACV supposed to stay A LITTLE OPEN when the solenoid is not connected ?? 

In other words, is the spring inside the IACV supposed to keep the valve completely closed?
Because that was the only thing that struck me perhaps as a little suspicious. The IACV was not completely closed when the solenoid was disconnected. 
Otherwise, the IACV did have a small film of carbon deposits but did not seem unusual. I cleaned it with carburetor cleaner anyway. 

I also cleaned the fast idle valve, since I was there, and I also took out the four fuel injectors and cleaned them, they did not seem dirty. I put things back together, but still, rough idle after about 20-30 minutes. 


4)	I DISCONNECTED BATTERY: 

And left it disconnected for 30’, just to give the ECU a chance to forget the … traumatic past. Engine was difficult to start afterwards, took 20” cranking before it started, very erratic and slow, then within another 30” it stabilized.

But still, erratic idle after about 30’.

Got another pending code P0171 (lean), but again, I think this might be because the new MAFS likely has different characteristics than the OEM part?


5)	I VACUUM PUMPED the EGR with the engine idling:

Engine stalled. So I guess that means the EGR is working? 

============================================

Any ideas? Especially whether the IACV is supposed to close all the way when the solenoid is inactive? 

Here is a video of the IAVC not closing completely
https://youtu.be/E9Z6HbqpHvQ


And here is a video shaowing me applying on and off voltage to the IACV solenoid (I connect/disconnect the electrical connector)
https://youtu.be/7XAklJn3vQM


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## jdg (Aug 27, 2009)

1) Easier way to do this is to spray carb cleaner with the engine running...if you can get it to idle smoothly...which it sounds like you can't at the moment...so, nevermind 

2) Voltmeters do NOT have what I would call a decent response time. Even a good one is only going to update the screen at about 10 times per second, generally less with cheaper meters. If you've got a decent Fluke brand meter with a bargraph at the bottom, that bargraph updates much quicker, but still won't catch the tiny fluctuations. About the only way to check for small fluctuations is with an o'scope.

3) Don't know this one for your model. In general, key off, IACV should be closed.

4) Disconnecting the battery on OBD2 vehicles may or may not work to "forget the traumatic past". Yes, it worked back in the early days of computer controlled cars. Not so much lately.

5) Yep, you got it. Open the EGR, dumps raw exhaust back in thru the intake, engine stalls, or at the very least tries to die.

I think you're might be headed for a fuel pump. Seen lots of cases where everything runs ok early in the day, then goes to crap later in the day after the fuel pump has been run for awhile, and your "lean" code would seem to support that. Only way to tell for sure is to put a fuel pressure gauge on it and watch it...or bite the bullet and swap it regardless.

P.S. I don't own a Pathy, nor have I ever worked on one. Just general engine knowledge.
(And I did stay at a Holiday Inn Express last night  )


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## 98Frontier4WD (Jan 12, 2015)

Thanks jdg,

1)	Yes, carb cleaner for vacuum leaks …I have not tried that but I did not mention that I did try the similar propane method. But it does not seem to work. I started by releasing some propane directly into the throttle intake, to see what effect on engine RPM I might expect, but there was almost no effect. It’s as if the ECU immediately senses the extra fuel and immediately readjusts, so there’s not really any noticeable change in RPM – especially since the idle is already uneven. So, I figured, if it does not work with all the propane directly into the intake, what’s the chance I’ll be able to detect a small leak? Also, I’m not sure how much propane is needed. I just used a small plumbing torch on up to max flow (what would essentially make a say 4 inch flame if you lit it). Maybe this propane trick only works on older cars? 

2)	I know that I cannot really see signal variations with a multimeter. But… my old MAFS had enough of a noise in its signal that I could actually see it even on a voltmeter (with the ignition on, engine not running and a piece of electrical tape over the MAFS air hole the signal should be steady – but with the old MAFS I could see variations on multimeter when I applied pressure on the MAFS connector or its housing) . Since that behavior went away with the new MAFS, I feel safe to assume that the noise was originating inside the MAFS – as opposed to some grounding issue or the ECU. Bottom line: The noisy MAFS may have been one additional cause of rough idle, but apparently not the only issue. 

I suspect t my old MAFS has some failing electrical connection inside (solder gone bad). But it does not really look like you can open these things, or at least not without risking destroying them. Without having another MAFS I did not want to risk destroying my only semi-working one. The car becomes barely drivable without a MAFS. Now that I have a secondary MAFS, I may attempt to fix my original OEM part -- especially since the new one does not seem to have the original OEM characteristics (though apparently ECU “learns” to interpret its signal – hence perhaps the initial P0171 “lean” code – but always as a pending code, never a check engine).

BTW, thanks everyone for all the help. Though the problem remains elusive, I feel I’m learning a lot. Hopefully this discussion is not so far off track to be useless to others.


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## jdg (Aug 27, 2009)

1) Just like you're thinking. The ECU sees the propane and adjusts quicker than you can detect it with your ears. The trick is to force open loop operation, keep the computer from adjusting anything. Disconnect the front O2 sensor or maybe the TPS. That should kick a code and keep the ECU in open loop.

Propane is a nice gas substitute. Carb cleaner isn't...which is why I still use carb cleaner to find leaks.

2) Yep, probably had a MAF with a bad circuit board, or at least a rotten solder joint in there. Relatively common, as far as oddball stuff goes anyways. And ya, usually not open-able without wrecking it.


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## 98Frontier4WD (Jan 12, 2015)

STARTED GIVING ME AN OCCASIONAL PENDING CODE P0171 (LEAN) :

Of course now I don’t know if this might be caused by that non OEM cheap MAF I’ve installed (since my original Hitachi MAF seemed to have a noisy signal). But in any case,

I CHANGED THE FRONT O2 SENSOR, no effect:

Thought perhaps a failing sensor would give a lean signal, causing the ECU to exceed acceptable fuel trims and flag an occasional pending P0171. Besides that sensor had 155K miles on it, not a waste changing it, I thought. But since it had no effect, I tried to look again at vacuum leaks,

TESTED FOR VACUUM LEAKS WITH CARBURETOR CLEANER:

Used carburetor cleaner this time, and tried to get the ECU to run in open loop for the test.
I thought I could cause open loop operation by disconnecting a few sensors. So I disconnected the Throttle Position Sensor, the MAF and the front O2 sensor. But the engine refused to run with all three sensors disconnected. I had to re-connect the MAF and with TPS & O2 disconnected it seemed to run, open loop I assume? I could now detect some change in engine pitch when I sprayed carb cleaner directly into the intake.

I sprayed around several places throughout the intake, but because the idle was erratic in the first place, my test was rather inconclusive. There was a rather non descript area where if I sprayed I could somewhat detect a change in engine RPM, but not always. Also, it is possible that the carb cleaner simply evaporated from where it was being sprayed and then rose to be sucked in by the throttle intake airflow. Seems like in order to do a better test I’d have to put some tube / big hose on the throttle intake to make sure carb cleaner fumes don’t find their way in. Anyway, I have to get another can of carb cleaner to repeat the test with a better setup.

BTW, I did open my original MAF, you can cut the black plastic cover (and hope to glue it back together if you manage to fix it). I figured there was likely some bad contact inside or a solder that had gotten loose. But the entire circuit is covered with a tacky sort of silicone or polyurethane gel. Even if I managed to get the gel out, the small wires that attach to the circuitry don’t seem to be soldered. They look like they are almost braised (or something) to the leads.

====================

Well, I have the fuel pressure test left top do. Then I’m probably ready to declare defeat. Have never taken my truck to the dealer, perhaps this is the time I find out whether they’re worth the $150 they want to charge me for diagnosis.


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## rogoman (Dec 16, 2004)

The best way to test for major vacuum leaks is to use a good vacuum gauge. To check the intake system for a vacuum leak, attach the vacuum gauge to a full vacuum source. With the engine fully warmed up, the reading at idle should be 18 - 20 InHg. At 3,000 RPM, it should be around 21 InHg.

If readings are under 18 InHg, check the intake manifold nuts to make sure they are tight. The gasket may have failed; spray a water mist at the gasket to see if the gauge reading changes. Also check the intake plenum bellows at the throttle valve and at the MAF for cracks or loose clamps.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Here are some vacuum gauge readings and their indications:
----------------------------------------------------------------------------------------------------------
Low & steady: Late ign timing/valve timing, low compression

Very low: Vacuum leak

High & steady: Early ignition timing

Gradual drop in reading from idle to higher RPMs: Excessive back pressure in exhaust system 

Intermittent fluctuation at idle: Ignition miss, sticking valve 

Needle fluctuates as engine speed increases: Ignition miss, blown head gasket, leaking valve or weak valve spring


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## 98Frontier4WD (Jan 12, 2015)

Thanks rogoman,

Well, maybe some data is finally coming up: 

I did a crude vacuum test. Below is a video of what is happening with my vacuum. Now I recognize this is probably not the right tool to test vacuum, tomorrow I’ll look for a proper automotive vacuum gauge for the job. As you can also see in the video I hooked the vacuum gauge to the brake booster hose which unfortunately has a valve -- so I had to resort to a hack where I put a paper clip to create some small vacuum release otherwise the valve would simply retain the maximum vacuum.

I see the needle fluctuating which is not a healthy sign, but it could also be an artifact of my gauge and setup. As I said, tomorrow I’ll look for a proper gauge and hook it up to a hose without a vacuum valve.

My Vacuum video: 
https://youtu.be/zoohhMJsnCg


Now since fast vacuum fluctuation could be a sticky valve, I did a google search and found this video:
https://www.youtube.com/watch?v=PI5bBV0ZFek

I did this same “paper by the exhaust” test on my truck and lo and behold, this afternoon that I had a sporadic rough idle issue (the idle is not always rough – it’s intermittent) what I saw was: The paper was being pushed back (normal) then at some point it would do a random suction (paper goes toward the exhaust-abnormal) and then immediately after the engine would try to stall -- but would not fully stall, there was a small surge towards more normal RPM and then the idle would recover till the next rough incident, which yesterday evening was about once every 5 seconds. But as I said this rough idle sometimes it’s not there at all, sometimes its mild and sporadic, sometimes is almost constant. 

Here is another video I took yesterday evening where perhaps you can hear the sound the exhaust makes when the rough idle malfunction is almost constant. Great Harley sound up close like that! I had not discovered the paper trick yet when I took this video so I don’t know what a paper would have done here. 
https://youtu.be/RQTGjkN_xlk

More tests tomorrow…


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## 98Frontier4WD (Jan 12, 2015)

Well, I’m still at a loss

VACCUM TEST:

So here is a better vacuum test. I connected the vacuum to the brake booster, but before the one way valve. 
In the video you can see the rough idle:
@ 0:05 Near stall
@0:38 Near stall
@0:53 & 1:30 I revved up to 4000RPM
@1:55 Briefly opened throttle ½ way
@2:42 Near stall
This is the first time I do a vacuum test. Does the little vibration in the needle qualify as unstable vacuum?
The tinging/sputtering vibration noise you hear in the video is not the exhaust but noise from inside the vacuum gauge, perhaps the needle fluctuating?

My Vacuum test: https://youtu.be/g7GcgtUDa9o


This next video shows idling only. As I noted down the times when a near stall occurs, I see they occur almost every 30 seconds regularly. Perhaps that has some significance?

My vacuum at idle only: https://youtu.be/oBqMvjeotjI


COMPRESSION TEST: 

Seeing whatever needle vibration I saw above, and seeing the paper sucked into exhaust in the “paper by exhaust” (like this guy shows on youtube: https://www.youtube.com/watch?v=PI5bBV0ZFek), and reading that it may indicate sticky valves, I decided to do a compression test. 

Test was done with all spark plugs removed, throttle full open, about 7 turns of the engine (cranking to 15 turns seems to yield 3 PSI more): 
Cyl 1: 162 PSI
Cyl 2: 162 PSI
Cyl 3: 158 PSI
Cyl 4: 158 PSI

I tried adding about 1tsp of oil to Cyl1 and got 170 PSI.

Nissan specs are: Standard: 178 PSI. Min: 149 PSI, Max acceptable difference between cylinders: 14 PSI.

So looks ok to me. No sticky valves? No bunt valves, no major ring and seal issues?

LOOK FOR BLOWN GASKET:

Oil looks fine, not milky
I opened the radiator and started engine cold. No coolant came out.

FUEL PRESSURE TEST:

I imagined that fuel supply issues would occur at high power, not idle. But since I have now pretty much checked everything, I went ahead and did a fuel pressure test. 
I Td a fuel pressure gauge after the filter and got:
36 PSI with vacuum hose attached to fuel pressure regulator (Spec: 34) OK
45 PSI with vacuum hose NOT attached to fuel pressure regulator (Spec: 43) OK

So, all systems are checking out ok

I now do get a pending P0171 (lean) code once in a while after letting it idle for a while. I also got a P0300 but I’m not sure about the conditions since I was doing all these tests with vacuum, compression and fuel pressures. 

In any case, when I took out the spark plugs to do the compression test, here is how they looked (engine had been rough idling for about 10 minutes before I took them out). 

I cannot see any unevenness, implying something might be wrong with one or more cylinders.


So what is going on?

As a reminder I have also: 
-Changed the MAF (ok with cheap non OEM part, since my original was noisy)—disconnecting MAF does not fix the rough idle anyway.
-Cleaned the Idle Control Valve and checked operation (spring by itself closes valve only ¾ of the way, but I assume that is normal?)
-measured the TPS resistance for linearity and the TPS switch
-Checked the fast idle settings (thermos-mechanical opening of throttle, throttle completely closed once engine warm)
-Checked the manual portion of the idle air (screw on throttle body) per shop manual procedure (If I repeat the procedure per shop manual I end up in the same ½ turn of the screw open-same as it was)
-Checked ignition timing per shop manual procedure (look for 20 deg BTDC with TPS disconnected) -- in case timing chain had skipped.
-Hand pumped the EGR valve at idle to verify that it did indeed cause engine to stall—in case EGR stuck open at idle.
-Changed front O2 sensor—in case it was about to fail and giving erroneous lean
-Put Techron in gas tank—to clean injectors
-Opened injectors to look for dirt—very little found
-Checked for vacuum leaks (not very thoroughly but vacuum measurements look good, don’t they?)

So what’s left:

The only thing I have not checked is ignition. But obviously I do have spark, so any static or simple continuity tests on ignition and coils are unlikely to reveal anything. Besides what sort of ignition issue is it that shows up only at idle? I yanked on cables, banged on distributors, disassembled distributor cap, nothing, no difference. Any ideas on the ignition? 

A mystery remains, that “Paper by the exhaust test” which at my exhaust does indeed show the paper being occasionally sucked towards the exhaust pipe when engine gets into its funky idle spurt (by is this cause or effect of the fact that engine is in near stall?)

Driving me nuts !!

As an added irritation, I have to pass a smog test by the end of the month…


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## Roader (Nov 9, 2014)

Can you isolate the miss to a particular cylinder?


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## rogoman (Dec 16, 2004)

98Frontier4WD said:


> So here is a better vacuum test. I connected the vacuum to the brake booster, but before the one way valve.
> In the video you can see the rough idle:
> @ 0:05 Near stall
> @0:38 Near stall
> ...


The small amount of rapid vacuum gauge needle vibration at idle shouldn't be there which might indicate sticking valves but more so dirty fuel injectors. Removing the injectors and looking at them doesn't prove that they are dirty or not. The dirt would be inside that you can't see.

You may have leaking fuel injectors or dirty injectors causing an uneven spray pattern. To test them, first disconnect the ignition wire from the coil, then unbolt the fuel rail from the intake manifold and pull off the entire assembly. Keep the fuel hoses still connected and don't remove any individual injectors. Also keep the electrical connectors on the injectors. With the assembly now away from the intake, turn the ignition key to the run position WITHOUT STARTING THE ENGINE. Now observe each injector to look for leaks. There should be no drips.

You can follow this up by having someone trying to start the engine while you observe each injector spray pattern; the spray pattern for each injector, of course, should be identical and uniform. 

Make sure you have at least 4 spare insulator rings for the injectors in case some are damaged during rail removal


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## 98Frontier4WD (Jan 12, 2015)

Thanks. I put the injector test in the queue… as far as pinpointing a particular cylinder at fault, no the only codes I have gotten occasionally are P0171(lean) and P0300 (random misfire) and neither compression tests or spark plug appearance show any difference between cylinders

... meanwhile,

SMOG TEST PASSED:
Well, it passed the smog test. So now at least I have time to fix this.
Here is a snapshot of the smog test results:


OCCASIONAL P0171 and P0300:

Engine still works well, including reasonable idle while cold and well after it has reached operating temperature.
But once it has been running for about half hour, the rough idle starts.
I’ve been monitoring for codes and when it starts rough idling, within about 5-10’ I often get a pending P0171 (lean), but not always. Typically I don’t get the code again in subsequent trips so Engine light typically never comes on and the pending P0171 quickly clears (one or two trips, I’m not sure). Sometimes, together with the P0171 I’ve gotten also an occasional P0300 (random misfire). 

drive with MAF DIDSCONNECTED test:

Another interesting fact: Rough idle is there even if I turn on the AC. However, if I disconnect the MAF then the rough idle is still there, but less than with the MAF connected. More interestingly, with the MAF disconnected when the AC compressor is on the idle is now steady at about 1000RPM.


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## Roader (Nov 9, 2014)

"...as far as pinpointing a particular cylinder at fault, no the only codes I have gotten occasionally are..."

Yet according to the video the truck seemingly misfires at idle. If it were me I'd yank the spark plug wires off one at a time, and ground it, start the truck, and see if I could isolate the misfire to a particular cylinder. That's pretty much what the self diagnostics does: a cylinder balance check.


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## 98Frontier4WD (Jan 12, 2015)

Well, I did capitulate and have taken the truck to two more garages now. But no success. Nobody seems to be able to find anything.

I guess the dealer is next in line to prove their worth. Hopefully I will not have to just pay for overpriced attitude.

To their credit, the two garages did not want to charge me anything since they were not successful, though I insisted on paying them $40 for the one hour or so they spent making an honest effort.

If I heard correctly, one of the mechanics told me that my Long Term Fuel Trim is at about -10% while my Short Term Fuel trim is about +24% (almost maxed out, I guess?). That seems puzzling to me. At a certain RPM after a while the LTFT should start adjusting so that STFT goes back down closer to 0. Not?

I'm thinking I should buy one of those ODB2 bluetooth or wireless tools and then download one of the many available programs (I heard Torque was good?) to verify these things myself. Anyone has any experience with those?


The garages also double checked many of the items I had originally investigated, including smoking the whole system in search of vacuum leaks but did not find any. In one mechanic’s opinion, the lean condition could cause the vacuum needle to vibrate a bit, there is not necessarily something wrong with the cylinders, valves or gasket, he said, things that were more or less excluded by the other tests anyway.


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## jdg (Aug 27, 2009)

This is one of the bluetooth OBD adapters I've got (have 3, all in the same style case, different labels, etc.). Can find them on Amazon as well.

http://www.ebay.com/itm/-/310678002251?ssPageName=ADME:L:OU:US:3160

"Torque" is good enough. I use the free "Lite" version. Works for me.

Not sure about the Nissan's, as I haven't had the opportunity to try it, but... If I clear the codes on my Vibe, the LTFT and STFT values get zero'd out and the ECU basically starts from scratch. Even the idle gets stupid and I have to get it out on the highway to let the 'throttle by wire' reset itself.

You're thinking right about your LTFT and STFT trims. But, if you haven't driven the vehicle long enough, the ECU won't have a chance to do what you're wanting. Takes awhile to get the LTFT trims to adjust themselves. The STFT gets updated almost immediately based on O2 sensor feedback, while the LTFT gets adjusted only after a complete drive cycle has been completed and even then only after X amount of time, miles, etc. has gone by.
In the absence of of really good hard data and all other things being equal, your STFT does suggest a lean condition, while the LTFT could just indicate an older engine.
Bouncing needle on the vacuum gauge--It's a 4 cylinder engine. If you've got an 'undamped' vacuum gauge, the needle is going to bounce at idle. Just the way it is. You end up kinda smoothing the needle out in your mind.

Next step might be to pull the injectors and do an injector leak down check. Pull the injectors out, put a cup under them, turn the key on/off a few times (pressurize the fuel system), let it sit, see if the injectors drip into the cup. That'll rule in/out a leaky injector. 

After that, you get a fuel injector pulse tester. Basically, you pull the injectors one at a time, hook up a fuel pressure gauge to the rail, hook up this pulse tester, turn the key on/off a few times (again, pressurize the fuel system), pulse the fuel injectors with the tester, which pulses the injectors X amount of times for a known time, and you read the fuel pressure gauge to see how far the fuel pressure dropped. Each injector should drop the fuel pressure by X amount of psi, within a certain tolerance. Any injector that doesn't drop the fuel pressure enough isn't shooting enough gas. Any injector that drops the pressure too much is shooting too much gas.


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## 98Frontier4WD (Jan 12, 2015)

Thanks jdg. I ordered something similar waiting on it…

Sorry for the long silence. Had to get some business travel out of the way.

Took the truck to the dealer first time Friday. But their diagnosis seems vague to me and leaves a lot unexplained.
Here is how it went:

I talked to the master tech, gave him a two page summary of symptoms and what I had already done and spent 10’ explaining verbally.
As I was talking he told me I should have done an “idle learn” after changing the MAFS. But I don’t think this ’98 Frontier has an idle learn procedure. I did not find one mentioned in the Service Manual. In any case, I cannot expect techs to know by heart all the models and years that have an idle learn procedure. He did not mention doing idle learn himself afterwards.

He later called me and thought it may be a short or something of that nature. He said the MAFS signal was really noisy “I have never seen it that bad, and wonder how does this truck even run?” were his words. He was then nice and sent me the diagnostic page through email. Here is what I got:



BTW, I’m not sure I understand the two middle signal traces (especially the Air/Fuel Alpha, isn’t that value too low? ), so if someone has something to add I’d appreciate it. 

Sure, looking at that first trace, seems like there’s virtually *NO* MAFS signal. That trace looks to me more like the signal from Eddie Van Halen’s guitar amplifier.

So first mystery is how can the truck even run without major drivability issues (other than a rough idle) with a MAFS signal like that? (No MAFS codes from ECM) 

Second, this does not corroborate at all with my measured MAFS signal. Below are the videos showing my voltmeter measurements. Of course it could still be that the noise is so high frequency (looks to me in the order of 10HZ ~100msec period) that I cannot pick it up with my analog or digital voltmeters. However, as you can see in the video, the MAFS seems to respond to changes in engine load as expected and is also within voltage specs. I also blew into the MAFS with ignition on, and it does indeed seem to respond as expected. 
I’m thinking that the noisy signal trace I got from the dealer shows a MAFS that should be totally unresponsive, not what I observe.

In any case, here are my videos. To rule out problems in the wiring harness, I took out the ECM and measured the MAFS signal directly at the ECM input, as shown here:


First is video of measurement using a small analog voltmeter:
https://youtu.be/vdCYtiSyeIc

As you can see my video shows:
@idle my MAFS signal = 1.3V (spec is 0.9-1.8V) 
@2500RPM no load my MAFS signal = 1.9V (spec is 1.9-2.3V)
@3500RPM no load my MAFS signal = 2.3V
The signal seems to increase linearly with increasing RPM – as expected. 

In this second video I repeated the measurement with a digital voltmeter:
https://youtu.be/-jb_MRQKADM
@idle my MAFS signal = 1.40-1.45V (spec is 0.9-1.8V)
@2500RPM no load my MAFS signal = 1.97V (spec is 1.9-2.3V)
@3500RPM no load my MAFS signal = 2.35V
Snapping the gas (higher engine load) brings my MAFS signal up to 3.5V


As an extra test I just turned on the ignition (engine not running) and just blew into the MAFS hole in the throttle intake. As you can see the airflow registers as expected. 
https://youtu.be/5ylCFTawZtk

The dealer’s suggestion was to replace the MAFS with a genuine Nissan at over $500 – just to continue testing. I already paid him $150 for this diagnosis.

I’m not expecting this cheap MAFS I put to be that good. I’m sort of expecting skewed long term fuel trims as the MAFS is likely to have a different airflow/signal characteristic. But I do expect the truck to run and idle with the fuel trims taking up the slack. My hope was to confirm that the original MAFS was the culprit and THEN go ahead and try to find an OEM part. But the evidence makes me doubtful that the MAFS is indeed the cause of my problems.

The biggest discrepancy is that I see nothing like what the dealer sees in the MAFS signal. Of course he’s (presumably) measuring the MAFS as the ECM sees it, while I’m measuring the signal at the ECM input. If there is a disparity indeed, that would point to a bad ECU. An ECU whose only malfunction is to misinterpret the MAFS signal. What is the probability of that? Even taking into account the fact that I should start looking at oddball things at this point.

I guess next thing would be to borrow a scope again and see if I can record the MAFS signal the dealer sees. 

P.S. Will I be able to see traces like the one the dealer sent me with torque? Or at least quickly updating real time values?


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## jdg (Aug 27, 2009)

98Frontier4WD said:


> Took the truck to the dealer first time Friday. But their diagnosis seems vague to me and leaves a lot unexplained.
> Here is how it went:


Overall, in this post, looks like the guy is actually trying, vs. trying to pull the wool over your eyes.



> Second, this does not corroborate at all with my measured MAFS signal. Below are the videos showing my voltmeter measurements. Of course it could still be that the noise is so high frequency (looks to me in the order of 10HZ ~100msec period) that I cannot pick it up with my analog or digital voltmeters. However, as you can see in the video, the MAFS seems to respond to changes in engine load as expected and is also within voltage specs. I also blew into the MAFS with ignition on, and it does indeed seem to respond as expected.


Gotta throw this out there.
A 10HZ (10 times per second) signal would be a signal that runs about 600 times per minute, much like a single cylinder engine running at 1200 rpm would be ingesting a gulp of air 600 times per minute.
See where I'm going with that?
If the MAFS was really sensitive, then that trace looks 'normal' as it is actually registering each and every 'gulp of air' when an intake valve opens up.
Now do I think that's realistic? No, probably not. A chunk of wire in a hot air style MAFS can't possibly change temperature that quickly. Certainly is plausible though.



> P.S. Will I be able to see traces like the one the dealer sent me with torque? Or at least quickly updating real time values?


No. Your ECM probably runs on the ISO9141-2 protocol (most '98 Nissans do).
Assuming you turned off every other parameter and only leave the MAFS input on, at the very most you'll get around 10x/second updates, on a good day.

I think the MAFS is a red herring and leading you down the wrong path. Sure, could be the root of the problem. I don't think so this time. Especially since you've gotten the same results with 2 different ones.

Am I to assume the fuel injectors themselves haven't been checked yet? Leakdown, flow rate checks, etc.?


Also, on a side note, if you take those meters, put them on DC, check the voltage, then put the meter on AC and check the voltage again, you'll be able to tell approximately how much your voltage is bouncing.
For instance, if you read 1.4V DC at idle, then switched over to AC and got .2V, you can be relatively confident that you signal is actually a jumpy signal bouncing between about 1.2V-1.6V.
It won't be exactly those values as most meters don't work over a wide range of frequencies in the AC mode (unless you've got a good Fluke or other brand name), but it'll give you a ballpark. Similarly, if the reading on the AC scale is close to zero, you can also be confident that the signal is a steady one that's not varying by a whole lot.


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## 98Frontier4WD (Jan 12, 2015)

I JUST CANNOT SEE ANY NOISE ON THE MAFS SIGNAL WITH A SCOPE:

I went ahead and measured the MAFS signal entering the ECU (pin 54) with a scope. 
I cannot see anywhere the noise that the dealer sent me (see my previous post).
Here is my scope trace for MAFS signal entering the ECU:



The scale is 2seconds per square the voltage 0.5V per square. It shows me revving up the engine from idle.
I cannot tell if the voltage levels are correct (they are within spec), but the behavior is as expected. You can see how the signal surges by depressing the accelerator. Certainly I do not see the noise. The signal looks quite smooth. Definitely nothing like what the dealer sent me ? ! ?

Here is one of the videos I took showing the scope trace:

https://youtu.be/dUm6E1512tU

And here is a video of the MAFS signal I made during rough idling. You can actually see how as the idle drops below the pre-programmed 850RPM, the ECM opens the IACV (I can hear this when the air filter is off) in an attempt to bring the idle back up (presumably injects more fuel too) and the increased airflow is seen in the increased MAFS signal. 

http://youtu.be/r4VBFGXGxok


INDIVIDUAL CYLINDER AIR INTAKE VISIBLE?

Jdg, your intuition on individual cylinder air intake being picked up by the MAFS might be correct. Interestingly, the only high frequency I see is exactly when I snap the gas -- when the butterfly opens and the individual cylinder air gulps become large -- then I can see oscillation in the signal. I think these might indeed be the individual “gupls” of air intake from the cylinders, since the frequency of the oscillation is 2x the RPM. So I guess this hot wire MAFS seems sensitive enough to pick up some of these high frequency airflow variations !?! If anything, being a cheap one, I thought it would be slow in responding…

Here is the MAFS signal trace when snapping the gas:



And here is a similar video but with a fast trace, where you can see the signal oscillations when snapping the gas:

http://youtu.be/8saghlBiZYg

But other than those brief periods where the gas is snapped, I see no noise on the signal. Certainly nothing like what the dealer shop sent me. 

So bottom line…

If the dealer indeed sees noise in the MAFS signal when he reads it from inside the ECM while I don’t by reading it as it enters the ECM, there are three main possibilities:

a)	I am measuring wrong, but serendipitously read something that looks exactly like a reasonable MAFS signal
b)	The dealer is reading the MAFS wrong from the ECM
c)	The signal is ok entering the ECM but there is noise that scrambles it in the internal ECM signal conditioning, or the analog to digital converter, or something like that

a) seems quite unlikely. So does b). As I understand it, there is nothing to connect that the dealer tech might have gotten wrong. You just connect to the NISSAN consult port and the Nissan diagnostic just reads the MAFS digital value from some predetermined location in memory. Does not seem like there’s much opportunity for error. 

c) would imply a bad ECM. A bad ECM where the only malfunction is scrambling of the MAFS input signal. Perhaps I’m biased but I’m also very skeptical of diagnoses that blame the ECM.

In any case,
To test this last scenario c) I wondered if I could just feed an artificial constant MAFS signal to the ECM. If the ECM is scrambling the signal then it should scramble my constant voltage too, and continue to run rough, or worse. So I just used a potentiometer as a voltage divider to create a constant voltage of about 1.7V (what the MAFS typically puts out at idle). I disconnected the MAFS, and fed the 1.7V signal directly into the ECM MAFS signal input (pin 54). THE ENGINE IDLED BEUAUTIFULLY. If the signal were getting scrambled, I should still have the rough idle, shouldn’t I? Of course with this artificial signal you cannot do much other than idle. As soon as you press the accelerator the engine stalls. Poor ECM sees no change in the airflow, probably does not increase the fuel and thus with throttle open the engine stalls. I’m pretty sure the ECM gets thoroughly confused by that. 

Another thing I feel this proves is perhaps the absence of anything mechanically wrong with the engine, since I got it to idle beautifully by simply feeding it an artificial constant MAFS signal. 

WIRELESS OBD2 READER:
Anyway. I got the wireless OBD2 reader today. This one: 






I have not connected it yet. I was hoping to get it to work with my iPhone but none of the 4 free apps I tried seem to work that well (dropping data etc). I’ll keep looking or may have to resort to using laptop computer. I don’t mind buying an app for this, but I first want to see the limited functionality demo version work, which has not been the case yet.

Yes, nobody has looked at the injectors yet. 

This is becoming quite the intriguing problem, I must say…


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## jdg (Aug 27, 2009)

You won't see any noise at such a slow sample rate. The earlier 'scope shots were sampled at 100Hz. You're sampling at .5Hz. Won't see anything there except an overall snapshot. Noise won't show.



> I cannot tell if the voltage levels are correct (they are within spec), but the behavior is as expected. You can see how the signal surges by depressing the accelerator. Certainly I do not see the noise. The signal looks quite smooth. Definitely nothing like what the dealer sent me ? ! ?


See note above.
As far as voltage levels being correct, they can be off quite a bit and the computer will compensate up to a point (fuel trims).



> Jdg, your intuition on individual cylinder air intake being picked up by the MAFS might be correct. Interestingly, the only high frequency I see is exactly when I snap the gas -- when the butterfly opens and the individual cylinder air gulps become large -- then I can see oscillation in the signal. I think these might indeed be the individual “gupls” of air intake from the cylinders, since the frequency of the oscillation is 2x the RPM. So I guess this hot wire MAFS seems sensitive enough to pick up some of these high frequency airflow variations !?! If anything, being a cheap one, I thought it would be slow in responding…


I was guessing. Could be that MAFS's are fast enough to catch each intake stroke. I've never 'scoped one myself, just watched the values coming out of the OBD port.



> But other than those brief periods where the gas is snapped, I see no noise on the signal. Certainly nothing like what the dealer shop sent me.


...because you aren't sampling fast enough. If your 'scope is capable of it, bump up the sample rate, do a capture, eventually you'll see the same 'noise'.



> a)	I am measuring wrong, but serendipitously read something that looks exactly like a reasonable MAFS signal
> b)	The dealer is reading the MAFS wrong from the ECM
> c)	The signal is ok entering the ECM but there is noise that scrambles it in the internal ECM signal conditioning, or the analog to digital converter, or something like that


a - looks like a good MAFS to me
b - mechanic sounds like he's got good intentions with bad execution.
c - doubt it. If that was the case, it would run like serious crap at high revs.



> a) seems quite unlikely. So does b). As I understand it, there is nothing to connect that the dealer tech might have gotten wrong. You just connect to the NISSAN consult port and the Nissan diagnostic just reads the MAFS digital value from some predetermined location in memory. Does not seem like there’s much opportunity for error.


Except for the mechanics interpretation of the data.



> In any case,
> To test this last scenario c) I wondered if I could just feed an artificial constant MAFS signal to the ECM. If the ECM is scrambling the signal then it should scramble my constant voltage too, and continue to run rough, or worse. So I just used a potentiometer as a voltage divider to create a constant voltage of about 1.7V (what the MAFS typically puts out at idle). I disconnected the MAFS, and fed the 1.7V signal directly into the ECM MAFS signal input (pin 54). THE ENGINE IDLED BEUAUTIFULLY. If the signal were getting scrambled, I should still have the rough idle, shouldn’t I? Of course with this artificial signal you cannot do much other than idle. As soon as you press the accelerator the engine stalls. Poor ECM sees no change in the airflow, probably does not increase the fuel and thus with throttle open the engine stalls. I’m pretty sure the ECM gets thoroughly confused by that.


That's a helluva bit of troubleshooting  
If you're thinking that a rotten input signal is the problem, then put a low pass filter inline with the MAFS signal...
MAFS signal -> 100 ohm resistor -> ( 10uf capacitor to ground ) -> ECM (kinda looks like a T, resistor goes before the capacitor, - on the cap goes to ground).
100 ohm resistor and 10uf cap gives you a rolloff a bit below 100Hz dropping to practically nothing above 1Khz.

Or...try wiring in a hard ground wire from the MAFs connector to ground. The ground wire might meter out good (eg. close to zero ohms, etc.), but might be hanging on by one single conductor in a piece of stranded wire somewhere and won't carry any load. For that matter, might want to try hard wiring in the signal and power wire as well.

While I'm thinking about it, ever build a wire load tester using a tail light bulb?
If you end up thinking it's a rotten wiring problem, get a small 12v battery, disconnect the wire at both ends, and try to run enough juice thru the suspect wire to make the bulb light up. Like I said, a wire might continuity test good, but wouldn't be able to flow any current. Sorta like a garden hose that's almost completely plugged. It ain't leaking, but you can't get any water thru it.


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## 98Frontier4WD (Jan 12, 2015)

jdg, thanks for hanging on with this long thread. More information is coming.

Regarding the scope trace, the time shown (2seconds to 500msec) is the scale, I.e. the time per square on the display, not the sampling rate. The sampling rate is much higher. So you would see noise even on a slow trace. It would just all smudge together.

I did the ground current carrying capacity test (tail light bulb) you mention for the MAF signal shielding (see post #13). I have not done it for the MAF ground itself. 

GOT SOME NEW DATA WITH OBD2 WIRELESS DONGLE: 

I gathered quite a bit of sensor data on a few trips using the new OBD2 dongle in combination with the OBDCarDoctor free app on my iPhone. It is very useful.

I’ll have to process the data and then I’ll post it. I may have finally isolated the issue, but there are also a lot of things I cannot explain… could use some help… let me process the data and I’ll post again…


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## jdg (Aug 27, 2009)

98Frontier4WD said:


> So you would see noise even on a slow trace. It would just all smudge together.


That's what I was getting at. Can't see the trees in the forest.
We're on the same page here.



> I gathered quite a bit of sensor data on a few trips using the new OBD2 dongle in combination with the OBDCarDoctor free app on my iPhone.


Again, since the '98 is running the ISO9141-2 protocol, even at the highest sampling rates, assuming you're only pulling one single parameter, on a good day, you won't get much more than 10/second. Not that that's a bad sample rate, but it sure ain't 1000/second.

It's gonna turn out to be something stupid like a tail-light bulb loose


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## 98Frontier4WD (Jan 12, 2015)

Well, here is the OBD2 data:
Refresh rate is about 4 samples per second. Good enough for this type of analysis...


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## jdg (Aug 27, 2009)

That there is some troubleshooting data right there...yep...that's what I call it.

More later on your data. Long night at work.

Yep, not a lot makes sense up there in the captures, except when it does make sense.
I think you're looking at rotten wiring. The spiking on the MAFS signal is acting a bit like a what I used to get in fuel quantity systems on aircraft with broken signal wires or rotten shields. All kinds of goofy stuff. And if you put a meter on it or a 'scope, the signal itself would magically (capacitive coupling) go up and down by itself with zero inputs (either mechanical or electrical) otherwise.
I think my next step would be to let it sit and get overnight cold. Go out the next morning, hook up the OBD/datalogger, turn on the key and let it sit for an hour. See if the MAFS magically starts spiking by itself. My thinking is that there's a single rotten wire in the harness that's getting warmed up, possibly expanding, shifting, moving somewhere somehow.
After that, next step might be to cut the MAFS wires, one at a time, and splice in your own wire. Start with the grounds/shields, then powers, and do the signal last.
Maybe easier than that would be to add a 100K resistor across the signal wire to ground. If you're getting anything capacitive shooting thru to the signal, that should be enough to dampen it out. If not 100K, maybe a 10K.
Analog-Digital converters (such as the types inside ECMs) are high impedance inputs, which means they hardly draw any current and it's very to induce a bit of 'extra' current into a wire and make the voltage spike upwards (or downwards).


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## jdg (Aug 27, 2009)

Notes on the graphs...

Startup/warmup...
LTFT not 0 - just reset codes ... Resetting codes doesn't necessarily reset the fuel trims. I used to think it did too. Apparently not the case.

Warmed up/idling...
No idea why the STFT is ramping up and down. The only thing that comes to mind is when it hits a limit (eg. 25%), it triggers a change in the LTFT, and the STFT resets to 0. And O2, yes, very lean, but may not be warmed up fully yet.

Pulling out/idling at stoplight...
The only other halfway reasonable thing I can think of with the ramping is the above explanation, or possibly that the ECU is trying to complete a 'drive cycle' (checking whether or not the O2 sensor actively responds to programmed changes in fuel delivery).

Continued driving & on highway & still on highway...
Looks normal there.

Still on highway #2...
Yes, deceleration = fuel cutoff as long as RPM (and I'm sure a few other conditions) is above X amount of RPM.

32 minutes in...
Maybe the ECM (and you) has completed a 'drive cycle' and the ECM thinks everything is 'ready' for anything (eg. everything has been tested).

After that...MAFS spiking...
I'm outta ideas other than the rotten wires thing.
Wouldn't it suck if you got 2 identically bad MAFS? Not totally out of the realm of possibilities.

Get an average reading of your MAFS output from OBD (in grams/second) after you fire it up, before it starts to go nuts. Let's just say it's around 10 g/s (just throwing numbers here, my example is NOT the correct number).
When the MAFS goes nuts, is it mainly dropping towards 0 or spiking up towards some ungodly number, as based on your initial observations of an idling MAFS value?
I can't tell what your actual MAFS numbers are from the pictures, and photobucket isn't any help either.

One other thing that makes me think you've got bad wiring (or another bad MAFS) is the fact that when you blow into it, it quiets down for a bit. The fact that you had to handle something...air filter, the MAFS itself, wiring, tubing, whatever, tells me that something got moved somewhere...thereby leading me in the direction of the rotten wires.


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## 98Frontier4WD (Jan 12, 2015)

Thanks jdg, those suggestions were quite helpful. 

ELECTRICAL CABLES:
Now that I have the scan tool with graphing capability I did try to yank, shake and disturb any cable I could, bang on the dashboard, shake the truck, the ECM etc. … while observing the MAFS signal through the OBD interface. No noise or disturbance was ever noticed. I repeated the test by placing electrical tape over the MAFS to make sure I had a steady signal, so that I could more easily spot any perturbation in the signal. If you don’t put tape over the MAFS opening you just see variations in natural air movement (eg. a breeze). 

THERMALLY SENSITIVE MAFS?
So, unable to blame the cabling I pursued the theory of a heat sensitive MAFS. So I took the MAFS out of the throttle intake, left it hanging by its wires, turned on the ignition and used a hairdryer to start heating up the MAFS, while monitoring the signal with the OBD2 app. I was hoping to see some noise develop at some point. The MAFS opening was taped, to keep the hairdryer airflow out of the signal.

So, in about 15’, I heated up the MAFS to probably 160-170F (the temperature I thought it develops during driving) and waited 15 minutes, but I saw no noise. I decided to heat it more (did not care about destroying a $30 MAFS at this point). So I put the MAFS in a glass jar (to keep the hairdryer airflow from dispersing) together with a BBQ thermometer. Temperature inside the jar climbed to 230F. I really don’t think it heats that much normally, but what that heck, I thought, enough trouble already. I must have waited a good 30 minutes warming up the MAFS, …and then finally voila the spiking signal started!!
I verified for about 10’ that what I was seeing was indeed spikes and noise. Yes it was!. Airflow as seen by ECM was spiking between 0 and 12gr/sec even though the sensor was blocked with electrical tape!! Then I let the MAFS cool. But it kept spiking even as it completely cooled!! I must have taken no more than 10’ for the MAFS to cool, but I waited and it kept spiking for a good 30’ – spikes with MAFS cold now!! 

I thought, “Could it be something else that is warming up and causing the spikes, perhaps a wire (jdg, your intuition about just turning on the ignition and waiting was quite insightful!) or the ECM?”The temperature I had to reach to get MAFS to spike seemed unreasonable, and the fact that spiking remained well after MAFS had cooled made me suspicious that perhaps some other element is heating up and causing the spikes instead of the MAFS itself.

So, without turning off the ignition and while this new MAFS was still spiking, I swapped it with the old Hitachi MAFS. No heat. I waited about 5’, got no signal spikes.

So, I put back the new MAFS which by now had not only cooled down completely but had also been disconnected for 5’. No heat. Within 1’ the new MAFS started spiking again!!!. I watched it spike for another 3’ and then,

I swapped it again for the old Hitachi MAFS. I waited about 8’ but did not see any noise/spikes. So, 

I put back the new MAFS, which had spent even more time cold and disconnected. Within a few minutes it started spiking again. Stone cold and spiking!! (to be exact, not completely cold, it was about 10F above ambient temperature – I think the hot wire and circuitry inside the MAFS generate enough of their own heat to keep the device a little above ambient temperature). I observed this (new) MAFS spiking for another full hour. 

As a last test, I put back the old Hitachi MAFS and waited one hour and 45 minutes. I saw no noise/spikes.

I did though verify the defect of the old Hitachi MAFS, whereby pressing on the housing generates noise on the signal. But it’s a completely different type of noise than the spiking I get with the new MAFS. So my original Hitachi MAFS does seem bad, but in a different way.

So it looks like a thermally sensitive bad (new) MAFS right? 

Two things though are not convincing. A) that it took so much time and so high a temperature to cause the misbehavior. But more importantly, B) The fact that even when the MAFS signal is misbehaving a voltmeter (analog or digital) shows the same voltage as when the MAFS is healthy!! It’s as if in spite of the spikes and noise, the “average” signal is still the same, hence the voltmeters show the same. That does not seem very convincing.

So to do one more test, the next morning I just turned on the ignition with the new MAFS and let it sit there for about 3 hours (thanks for the suggestion jdg!!), without heating anything. I figured that if it were the ECM (perhaps the DAC or other input signal circuitry) or some thermally sensitive wiring that was causing the problem, I should see spikes/noise after a while. But I did not see any noise/spikes.

I’d be happy to get a new MAFS and get this over with at this point -- and stop torturing you all guys here on this very helpful forum. But I’m reluctant to shell out the $550 the dealership wants just to get another Hitachi part that likely has a design or systemic manufacturing weakness. And if I order from another party, I’m afraid I’ll get a third MAFS lemon… which will then further drive me nuts for another month or two (though I think, if anything, I’ve now learned how to debug them – so thanks for the support, regardless of what happens I have learned a lot about how to debug contemporary engines).

I think I’ll get a more definitive conclusion when I get the MAFS to misbehave and can actually see the noisy signal entering the ECM with the scope. Problem is, it’s not that easy to borrow the scope. Apparently, when I looked at the signal with the scope the last time, I must have not waited long enough and the MAFS was not misbehaving yet. 

So what about that funny STFT behavior when the MAFS is working? That STFT ramping up and down rhythmically over a 30 second cycle? It could be that the noisy MAFS, when it does eventually happen, throws the ECM for a loop and teaches it bad habits which then it carries on even when the MAFS IS working well. Well, one thing at a time. The MAFS signal is definitively going crazy. Let’s fix that and we’ll see if there is anything else. 

Thanks and sorry this has dragged on for so long. Hopefully it’s not a total waste.


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## jdg (Aug 27, 2009)

98Frontier4WD said:


> Thanks jdg, those suggestions were quite helpful.


Dumb luck on my part 
20+ years of troubleshooting dumb avionics problems on old military aircraft gives a guy a number of chances to try oddball troubleshooting techniques (eg. heating/cooling wiring, letting it sit, etc.). Then again, sometimes you play the game of shotgunning parts (random swaps) until something changes.
And again, nice to see somebody else with a clue for a change vs. the usual pinhead asking about cold air intakes, fart can mufflers, or the next bigger size rims he/she can put on a vehicle.



> turned on the ignition and used a hairdryer to start heating up the MAFS


That's the way to do it! Beat it into submission!



> Temperature inside the jar climbed to 230F. I really don’t think it heats that much normally, but what that heck


I don't think 230 is out of the norm. Think sitting at a long stop light on a hot day with no wind after a hard highway run. Sure, you've got the radiator fan going, but that's just circulating the exhaust manifold heat around a bit.



> I must have waited a good 30 minutes warming up the MAFS, …and then finally voila the spiking signal started!!


Nice...finally a repeatable(ish) result.



> Two things though are not convincing. A) that it took so much time and so high a temperature to cause the misbehavior. But more importantly, B) The fact that even when the MAFS signal is misbehaving a voltmeter (analog or digital) shows the same voltage as when the MAFS is healthy!! It’s as if in spite of the spikes and noise, the “average” signal is still the same, hence the voltmeters show the same. That does not seem very convincing.


The pieces/parts inside the MAFS are probably filled with some sort of potting material, an epoxy of sorts. Takes a long time for the heat to penetrate to the guts...and also takes a long time for the heat to soak out of the guts.



> I think I’ll get a more definitive conclusion when I get the MAFS to misbehave and can actually see the noisy signal entering the ECM with the scope. Problem is, it’s not that easy to borrow the scope.


I'd be willing to bet if you got in touch with a local Ham radio operator in your area, one of those guys would have an old 'scope they'd let you borrow for a few days.



> So what about that funny STFT behavior when the MAFS is working? That STFT ramping up and down rhythmically over a 30 second cycle? It could be that the noisy MAFS, when it does eventually happen, throws the ECM for a loop and teaches it bad habits which then it carries on even when the MAFS IS working well. Well, one thing at a time. The MAFS signal is definitively going crazy. Let’s fix that and we’ll see if there is anything else.


Ya, I don't have a halfway decent answer for that. Like I said before, I was kinda guessing it had something to do with completing the 'drive cycle' where the ECM basically jockeys things around a bit looking for something else to happen downstream in the process of basically self-testing various items...but. Sure, might be that noisy MAFS.

Don't know how you'd go about it, but, search around for other vehicles that use the same MAFS, hit the junkyard for a 3rd MAFS. 2 MAFS having a similar defect, unlikely but possible. 3 MAFS having a similar defect? Now that's getting wayyy out there...even in my book.


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## 98Frontier4WD (Jan 12, 2015)

Well, I ordered a new MAFS. That is why I kept quiet. This time I went with the original Hitachi (or so I think, I’ll have to see what arrives).

In the meantime to offer some entertainment, I’m posting the OBD2 graphs I was able to plot.

As a reminder, I ordered a cheap OBD2 dongle from Amazon (see previous post) and paired it up with the “OBDCarDoctor” app on my iPhone. I paid the $4 to get the paid version of the app, which allows you to monitor up to SIX parameters simultaneously and also allows you to SAVE the values to a comma separated text file. 

Then with modest spreadsheet manipulations I imported the text file into Microsoft Excel and created a graph. I was monitoring 6 parameters (the maximum the tool lets you simultaneously monitor) so the update rate on each parameter had dropped to around 1.3 seconds, but the information is still useful, I think. 

The six parameters I monitored are: Throttle Position (TP), Engine Load (EL), Short Term Fuel Trim (STFT), Long Term Fuel Trim (LTFT), engine RPM, and vehicle Velocity.
It was a long graph which displays nicely on excel, here I’m posting it broken into 9 snapshots.

The abnormal behavior I can see is:
a)	How the STFT does this rhythmic ramping up and down between 0 and +25% at idle. It’s as if it’s seeking something but I don’t understand what. The mixture is very lean during idle (I verified by reading the O2 sensor) but LTFT does not seem to make an attempt to adjust to the fuel deficit. Not sure what the ECM is doing in this mode.
b)	How the mixture turns lean and STFT trends towards +25% on deceleration. But perhaps that is normal?
c)	How at the end of my drive, arriving home, I waited about 9 minutes addling in front of my house, at which point, a little over 42’ since originally starting the engine, the MAFS signal goes nuts, the idle gets quite rough, combustion turns rich etc. You cannot see the MAFS in the graphs (again 6 parameters is all it lets me log simultaneously and I had already looked at the MAFS so I was more interested in what was happening with the other parameters). As a reminder, on another drive it took about 32 minutes for the MAFS to go nuts. This last run was in the evening, when its cooler so seems like it took longer, consistent with the heat sensitive MAFS theory.

In any case, these graphs seem very useful. And all for just $24 total !! (~$20 for the OBD2 dongle and $4 for the app). Seems like anyone can do it now.


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## jdg (Aug 27, 2009)

A) Still, the only thing I can think of as far as the STFT ramping goes would be the ECM looking for a change in a downstream O2 sensor...part of the "drive cycle". The ECM trims for a rich mix, loads the cat up with fuel. Eventually the cat can't "store" the fuel and the downstream O2 sensor pegs rich. Then the ECM trims for a lean mix, emptying out the cat. The downstream O2 sensor pegs lean. That's how upstream (pre-cat) and downstream (post-cat) O2 sensors do their job. The upstream is supposed to wiggle (a wiggle you probably can't see because you can't scan fast enough to see it...it's a relatively fast wiggle) a bunch and the downstream O2 sensor is supposed to stay steady...if the cat is doing it's job of storing and releasing and "burning" HC's. If the upstream and downstream were wiggling at roughly the same rate, it would signal to the ECM that the cat isn't doing anything and would flag a code for catalytic converter efficiency. At least it is ramping at regular intervals, suggesting it is a programmed thing in the ECM. If it varied a bunch, I'd begin to think surely wasn't right...but this almost seems like it's normal.

B) Yes, on decel, that's normal. It's called 'deceleration fuel cutoff'. Sort of like built in engine braking. Totally normal. If you watch the flag that says open loop / closed loop, you'll see the ECM switch to open loop on long decelerations, like letting off the gas going down a long hill or on a highway off-ramp.

C) I like the jittery MAFS theory, even though it seems like it's been a serious pain in the ass so far .

Entirely possible that the whole thing has been overthought a bit and the whole problem has been the MAFS all along and there is nothing wrong anywhere else.


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## 98Frontier4WD (Jan 12, 2015)

****************************
* FIXED! I think,
****************************

Well looks like the new Hitachi OEM MAFS fixed things!!

…at least most things, there are still a couple of things that look at little abnormal, but I’m not sure if they are transient and might go away as the ECM learns the new MAFS. 

So, Friday morning I installed this third MAFS, this one an OEM Hitachi, which I had just got from eBay for $200 (the dealer wanted $550).

The idle got better immediately, both during engine warmup and when engine warmed up. ECM entered closed loop normally after 2.5 minutes of idling. I immediately noticed the STFT was not doing the 0 to +25% ramp any more, though I did have a strongly positive STFT (+15 to+20%) with an LTFT of 0%. I let it idle for about 6 minutes past the point it entered closed loop and the situation remained the same STFT= +15 to+20% and LTFT = 0% while idling. There was actually a bit of STFT ramping, but much narrower, between +15% and +20% and not nearly as periodic and regular as the one I had with the previous MAFS.

Then I went out on some traffic, and eventually cruising some on the Freeway. When cruising, I had a LTFT of +9.3% and a negative STFT between -7% and -14%, so total net trim was close to 0%. After about 15 minutes of cruising, both STFT and LTFT went closer to 0%.

Also, the idle trims changed eventually. Idle STFT started oscillating closer to 0% (between 0% and +3%), but LTFT at idle adjusted to a rather strong +9.38%

There is no crazy MAFS signal after 30-40’ and the associated rough idle. The idle is very smooth. Revving up under no load seems healthier too. The O2 sensor is doing its rich-lean-rich-lean cycling as one would expect. The engine’s low RPM torque seems to have improved too. Transitions from deceleration to idle and then reacceleration are smoother too. 

Unlike before, deceleration seems to be running open loop, as you suggested is normal jdg, with STFT going to 0.00% on deceleration. Full power (full open throttle) seems to be running open loop too.

All in all, drivability is good again!

The only thing I’m still a little unsure about: Why do I have this rather strong positive LTFT of +9.38% at idle. LTFTs at other engine loads seem to be closer to 0%. 

I’ll wait a few more days, see what other observations I can make, and then I’ll write a summary post, so that the next unlucky chap that is tormented by two successive heat sensitive MAFS does not have to read through all 40 posts of this tread.

Meanwhile, I’ll enjoy my newly rediscovered smooth ride for a few days!!!

Regardless of what other problems might remain, this is a significant improvement!!

Thanks!!


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## jdg (Aug 27, 2009)

98Frontier4WD said:


> Well looks like the new Hitachi OEM MAFS fixed things!!


Hate it when that happens...Or do I like it? I forget... 
2 MAFS's with the same stupid problem. Who would've thought.



> ECM entered closed loop normally after 2.5 minutes of idling.


Does your vehicle have heated O2 sensors? If it does, it should go into closed loop within a minute easily, even on a bone cold engine.
meh...let it ride. Good enough. My old '95 Grand Prix without heated O2 sensors goes closed loop within a minute in the middle of winter. But, an apples and oranges comparison here.



> Unlike before, deceleration seems to be running open loop, as you suggested is normal jdg, with STFT going to 0.00% on deceleration. Full power (full open throttle) seems to be running open loop too.


As they should. I think the numbers go to zero's because there aren't really any numbers for it to crunch in that situation.
STFT is generally what the ECM is thinking and adjusting for right now and added in accordingly. The LTFT is more permanent and is added in (or subtracted out) even under open loop situations...which is probably the only reason why a vehicle that drops an O2 sensor can still function fairly well.



> The only thing I’m still a little unsure about: Why do I have this rather strong positive LTFT of +9.38% at idle. LTFTs at other engine loads seem to be closer to 0%.


Tiny vacuum leak, slightly plugged injectors, fuel pressure a tad bit on the low side, altitude, temperature, gas quality. A hundred different things for that one. Although, I'd be more willing to go with the vacuum leak since at idle a small vacuum leak is a larger proportion of all of the air going into the engine, whereas under load, that small vacuum leak becomes an insignificant part of the whole. Regardless, until the ECM flags a code, I wouldn't worry about it. That's why they have fuel trims...to compensate for small variations in this, that, and the other thing, wear and tear.


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## Frontier4wd4cy (Sep 19, 2015)

*Thanks so much!*

I just bought a 98 Nissan frontier with the same problem although I have had no loss of power at speed. I found this thread this morning and just read through every post. I'm incredibly impressed with the level of troubleshooting that was implemented! You guys have given me all the tools (hopefully) to tackle this problem.

After owning a 95 Volvo 850 turbo for two years I can attest to how incredibly helpful good forums can be and since I don't think I've owned a car that doesn't have a good forum to follow. Seriously stuff like this keeps me from just banging my head against the wall on my own.

Thanks guys, keep up the good work.


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## 98Frontier4WD (Jan 12, 2015)

=================================================================
* SUMMARY

98 Frontier 4WD with two successive bad MAFS
=================================================================

Now more than a week after new (third) MAFS, truck is still running fine, so, as promised, here is my final summary…

If you are unlucky to have a bad MAFS (Mass Airflow Sensor) and have the further misfortune of buying another one (especially a non OEM) that has a similar problem…

…then you may start pulling your hair out, and in desperation may find this thread…

So to spare you having to read all 40+ posts on this thread, or at least to put those 40+ posts into context, here is the summary of what happened and how it may apply to a similar situation:



My truck’s idle had been deteriorating a bit, but nothing that someone would not be tempted to attribute to an aging vehicle (98 Frontier 4WD with 160k miles).

Then after a few months, finally, after a four hour drive, the engine started occasionally stalling on idle when warm. The idle also got more noticeably and obviously pathologic. Also, once in a while I would experience engine “hiccups” short ¼ second loss of power at highway speeds, but it was infrequent. A couple of hiccups and then ok for another ½ hour or so. 

What had happened, is that the MAFS (this was the original OEM Hitachi MAFS) signal had become occasionally unstable. The signal was spiking with noise when pressure was applied to the MAFS housing. The noise became more pronounced as the MAFS got warmer, hence engine idle was getting worse as the engine was getting warmer and warmer, and that is why it probably got to the point of stalling after a long drive on a hot day. Of course I did not know that at the time. This is a diagnosis I came up with later. Hindsight is 20/20.

So I took the truck to a local garage. It was 4th of July, I was 250 miles from home and this one man mechanic shop was the only open. Very nice fellow in retrospect, I owe him some money. He correctly diagnosed that the MAFS signal was noisy, but attributed the issue to the MAFS electrical connector, because pressing on the connector would cause the engine to stall, but sometimes only. He tried to fix the connector, and since finding a new connector on the 4th of July was out of the question he did not even want me to pay him. As I said, when I go back I’ll give him a gift card because although he was not able to fix the problem his initial diagnosis was 90% correct. 

I futzed around with the connector myself once I got home, spending lots of time on it (something the garage cannot afford) and came to the conclusion that the noise in the MAFS signal was coming from the MAFS device itself, not the connector. This is because when I applied pressure to the MAFS housing I could see the noise (with a voltmeter initially and then confirmed with a borrowed scope) and after many tries I discovered that the pressure sensitivity got worse with rising temperature (engine compartment warm). When I mean pressure, I mean a lot of pressure, probably 10-20 pounds of pressure on the housing. When pressed, the MAFS would create signal spikes, even when the engine was not running, simply with the ignition on.

I’m saying this because you may also have the same Hitachi MAFS, and it could be that this is one of the main modes in which they fail (speculation here, I’m only one data point). There is a lot of circuitry inside these MAFS and so it is possible that some connection starts going bad and becomes sensitive to mechanical and thermal deformation. BTW, the circuitry inside is submerged in some sort of gel, so attempting to fix such a device seems rather impossible (I tried).

Was I sure at this point that MAFS was the culprit for all my issues? No. Why? Because a typical test in these situations is to disconnect the MAFS, so that the ECM flags the device as bad and goes into safe mode operation. If that fixes the problem then it’s a bad MAFS. I did that, and still had a rough idle, and more or less the same rough idle that I had before, so I was skeptical that MAFS was the true culprit. In retrospect what was happening (I think) is that the MAFS was bad and so its bad signal had altered the ECM’s parameters (ECM attempted to adjust), so when I disconnected the MAFS, the ECM continued to run with those bad parameters and the idle was still rough (with a very lean mixture in retrospect).

But I did not know all this at the time and, more importantly, the OEM MAFS at the dealership was $550, so changing it on a suspicion seemed like a costly gamble. 

So, I reckoned, I’d just buy a cheap after market Chinese MAFS for about $30 and if that seems to more or less fix the problem, then I’m sure it’s the MAFS and then I’ll buy the OEM part, possibly online where the OEM device was apparently selling for around $250. 

Well, trying the cheap $30 MAFS turned out to be a bad decision. To make this story a little shorter, this cheap MAFS …WAS ALSO HEAT SENSITIVE, albeit in a slightly different way. Its signal never spiked (signal was normal and responsive to RPM changes) until the engine compartment got warm, after about 30-50minutes of driving, depending on traffic. Once it started misbehaving, the signal spiked a lot. But the overall behavior was similar to the original bad MAFS: The erratic behavior when the device finally warmed up would teach the ECM bad habits (make it alter its learned operating parameters in an attempt to adjust). Then, even when the device was operating correctly, the bad ECM parameters learned would still cause a rough idle (rough in a different way though).

At this point, I gave up and took the truck to a local but reputable garage close to my work. They were puzzled since they could not explain the behavior. In retrospect, they tested the truck as engine was warming up and then also when it warmed up, but did not wait the full 30-50’ that it took for the MAFS to misbehave (who knew at that time that it took a whole 30-50’ for this second cheap MAFS to start misbehaving). So what they saw was the messed up ECM parameters (eg. Long term fuel trim) and an ECM unwilling to adjust. Little they knew that these were the parameters the ECM was adjusting to when the MAFS was eventually going crazy after 30-50’.

I then took the truck to the dealership, where they apparently first drove the truck, giving the heat of the engine plenty of time to reach and warm up the MAFS. Then, when they took measurements, they saw a crazy MAFS signal and wondered how does the truck even run?

Hearing three different stories from three different garages with two different MAFS, I decided to test the MAFS myself with a scope. Unfortunately I did that before the 30-50’ warmup period and saw a normal signal. Just like the second garage did.

I continued to debug and finally, two months later I came to the final conclusion that both the original OEM MAFS had become heat and pressure sensitive and that the new MAFS was just heat sensitive. You can read the history details in the previous posts if you want.

I bought an OEM Hitachi MAFS online for $200 and all the idle problems went away. The overall engine behavior seems to have also reverted to 50k miles younger, better torque at low RPM and smother transitions from acceleration to deceleration and vice-versa.

Especially, 

Thanks a lot to all who helped with the problem, especially jdg who had the patience to hang around until the end! I might have given up, or would have at least taken me much longer to figure out without the guidance I got from this forum.


========================
P.S. I have one more problem turns out (small manifold crack seems to be affecting O2 sensor readings) but that is a completely different problem, so I’ll post another thread.


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## jdg (Aug 27, 2009)

98Frontier4WD said:


> here is my final summary…


Summary? How about "Short Story" ? 



> Thanks a lot to all who helped with the problem, especially jdg who had the patience to hang around until the end! I might have given up, or would have at least taken me much longer to figure out without the guidance I got from this forum.


It also helps out a bunch when the guy asking the questions (that be you) is willing to do the legwork, not to mention go way above and beyond, to troubleshoot the problem and has a decent understanding of "things" in general vs. trying to fit what "a friend says" or "he read somewhere on the intardnets that" to his/her particular situation.



> P.S. I have one more problem turns out (small manifold crack seems to be affecting O2 sensor readings) but that is a completely different problem, so I’ll post another thread.


Exhaust manifold cracks will do that. Those same pulses that mess with the MAFS of course propagate thru to the exhaust side of the house and can easily draw fresh air thru those cracks and try to pull the O2 sensor lean.


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## Gearbanger1 (Oct 25, 2019)

98Frontier4WD said:


> =================================================================
> * SUMMARY
> 
> 98 Frontier 4WD with two successive bad MAFS
> ...


I registered to be on this sight to thank you so much for your thorough troubleshooting and accounting of same. I was able to align your symptoms with what my father and I were witnessing on our truck. We then opted to purchase a new OEM MAF and the truck now runs like new. It is rare to see a thread of any kind where follow through is so thorough and complete. Thanks again for helping reduce our downtime!


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## 98Frontier4WD (Jan 12, 2015)

Thanks a lot. 
Happy to see that the thread is still providing useful advice.


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