# 1994 VG30E 4x4 SE Truck - Severe Lurching/Lugging at lower RPM; ECM reports Code 34, Knock Sensor



## tmoat (Jul 30, 2021)

Starting a new thread on this topic as it has deviated somewhat from where it was initially posted.

1994 VG30E King Cab 4x4 SE V6 truck, VIN 1N6HD16YXRC402885.

This relates to severe lurching/lugging, particularly at low RPM and in lower gears, Problem is evident while idling (occasional drops in RPM and audible ticking sounds from engine what that happens). Problem is apparent when engine is cold, and appears to worsen as engine warms up. I took the truck to my local Nissan dealership. They confirmed fuel pressure is fine at engine intake. They did not have diagnostic tools for a vehicle this old, so I brought the truck home and diagnosed the following:

ECM reports Knock Sensor error code, Mode 3 Code 34. No other codes reported.

I tried the "dollar bill test" mentioned in another thread on this forum. The idea is to place a thin piece of paper at the tailpiece outlet to see if the outlet pressure is strictly positive or alternating positive/negative. I observed my bill being sucked flat to the tailpipe every time the hesitation/lugging symptom manifests – i.e. there is a sharp vacuum pull in the tailpipe when RPMs drop.

I looked for leaks in the air intake duct and Mass Airflow Sensor connections - all look/feel good and solid. 

I confirmed continuity through the main wiring harness between the Knock Sensor connector pin E back to the ECM pin 27.

I observed open circuit when measuring through the KS harness from the connector pin E to the Knock Sensor. The manual indicates this should be short to ground - ergo Code 34. The KS wiring harness and connector terminals appear to be in good condition, well-wrapped, as far as I can see towards the back of the block. The KS is well-obscured by the intake manifold so I can't see the entire harness. The open-circuit appears to be a fault in the KS harness or the KS itself, I can’t tell which without cutting the harness open and knowing how to diagnose the sensor. I am not convinced this is the root cause of the problem, so I kept looking.

I loosened/tightened the grounding straps nearest the knock sensor connector to ensure good bonding. One screw was quite corroded, but able to take torque without failing. I measured continuity across the bond - seems OK.

I checked the impedance of the ignition coil. It seems alright - 1.1 ohms primary (pins A-B on LV side), 12.44 kohms secondary (pin A to HV output). The manual indicates the secondary should be "approximately 10k ohms”; I am reading 25% higher than that, but clearly there is a spark. HV wires are all NGK and all less than two years old. Distributor looks clean, but I did not pull out the whole distributor assembly to confirm that.

I confirmed the Power Transistor diode forward voltages are correct.


Thoughts:


KS wiring harness might have a break in it; relatively easy to check/repair for the accessible portion of the harness.
KS itself could have failed, but I don't know what its typical failure modes are (open? short? intermediate impedance?) and can't find conclusive information online regarding what to look for in KS failure diagnoses.
I am not convinced that the KS fault is the root cause of the problem; it could be reporting a symptom of the problem. I don't see any other error codes however.
I am thinking of replacing the KS as it has 170k miles and 27 years on it. That might not solve the problem, but it would rule out the KS as the root cause.
I doubt this is relevant, but a few days before this fault showed up I used high-octane fuel for the first time in a year. Coincidence, or could there be causation?

Advice requested from the experts:

Would a leak in the intake manifold cause the lurching/lugging problem?
If there were a leak in the intake manifold, would I expect any other error code other than the knock sensor to be reported?
Might the use of high-octane fuel be a factor?
Could this be a sticking valve?
If I were to remove the intake manifold and replace the knock sensor, is there any other preventative maintenance work I should do along the way?

In addition to the occasional hesitation/lugging, I hear a continuous "gurgling" sound from the engine that seems loudest between the front of the block and the radiator. It is the sort of sound a stone might make when rolling around in the bottom of a plastic pail. Water pump perhaps? The vehicle was just given the full L/O/F service routine at a Nissan dealership, so I would assume everything was lubricated that should have been, and no mention of this noise was mentioned to me. Any thoughts on this sound?


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## jp2code (Jun 2, 2011)

Quite an extensive diagnosis. 

The knock sensor is made of a piezoelectric crystal. When these crystals are compressed, they produce a voltage. There are no moving parts in the crystal, so it is very unlikely that it went out. As you suspect, the problem is typically in the wiring that goes to the knock sensor. 

I replaced my knock sensor when I had code 34, but it did not eliminate the problem. And replacing it is a lot of work. The intake has to come off because the knock sensor is bolted to the block between the two cylinder heads. 










I'd try going back to regular gasoline first. 

Next, try buying a knock sensor and testing the voltage and resistance to the one on your truck. Your truck will go through a harness while it is still installed, but the readings should be identical.


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## tmoat (Jul 30, 2021)

Hi Joe - thanks for the reply, and especially for the photo - now I know exactly where to look and what to look for. I appreciate your suggestions. 

Unfortunately, my tank is near full of premium. Maybe I could open the line before the fuel filter and feed the engine with a jar of regular and see if there is any difference. I have never heard of high-octane fuel being "bad" for combustion; Every year or two I have put in some premium on the belief that it would help clean the injectors and remove carbon deposits. 

I agree - I think the next step will be to open the harness and see if I can find a damaged wire. I can't get to the sensor without removing the manifold, but from your photo it looks like there is very little stress on the sensor connector or wire, whereas there are plenty of bends and merges near the connector to the main harness. If that doesn't fix it, then I think some surgery will be required. I will also buy a replacement sensor and see what sort of output I can get out of it with my oscilloscope. I'm used to connecting crystals to microprocessors (voltage input to mechanical output). This time it would be mechanical input to signal output. My guess is that the signal would be very small - maybe tens of millivolts. I will see if I can find a datasheet on the part number. 

Thanks again - Trevor


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## jp2code (Jun 2, 2011)

I got my BS in physics back in 1996. It's nice to see someone who knows what an o'scope is. 

If the truck runs, even crappy, with premium, just go for a long drive and burn it off. 300 miles, tops, if yours is like mine. 

My guess is that you could have carbon build-up in the cylinders or something. The extra octane is moving the optimal ignition point to where the knock sensor reading cannot be remedied by the ECU. 

Perhaps the Ignition Timing is off. When was the last time you put a Timing Light on it? The timing should be about 14 degrees.


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## rogoman (Dec 16, 2004)

tmoat said:


> Advice requested from the experts:
> 
> Would a leak in the intake manifold cause the lurching/lugging problem?
> If there were a leak in the intake manifold, would I expect any other error code other than the knock sensor to be reported?
> ...


Here's several things that could cause your lurching/lugging:

What brand of spark plugs are you using? You should be using OEM NGK plugs; other brands such as Champion or Bosch many times cause driveability problems in Nissan engines.
The cam position sensor may be marginal.
Incorrect fuel pressure. Tee-in a temporary fuel pressure gauge between the fuel feed hose and the fuel rail. 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

There may be a major intake system vacuum leak. 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.
Dirty fuel injector(s). Run some good injection cleaner, like BG products 44K, through the system; give the cleaner about a week or two to do it's job.

Here are some vacuum gauge readings and their indications:
----------------------------------------------------------------------------------------------------------
Low & steady: Late ignition 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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## tmoat (Jul 30, 2021)

smj999smj just posted this on the old thread, which I think is worth a try:
The knock sensor generates a voltage when impacted, so a dead knock sensor wouldn't cause the ECM to retard the timing. Typically, they almost never cause a drivability or performance issue with the exception of the supercharged VG33ER engines. They do go bad and they are a pain to replace because they are bolted to the top of the engine block and the lower intake manifold must be removed. You may want to look into relocating the knock sensor, as many do. A knock sensor and subharness from a 95-99 Maxima is used. The one end of the subharness plugs onto the knock sensor and the other plug is cut off. The wires to the knock sensor are located within the EGI harness and cut and the new subharness w/ sensor is spliced in. The knock sensor is then bolted to the engine, often at an intake manifold bolt location. It's not a sensitive as a location as being directly on the block, but it saves a lot of work and some money in parts. There's a thread about how to do it at ClubFrontier.org and there is probably one or more YouTube vids on it available. Some of the specs on the older Nissans can be a bit generic. 12.44 Kohms in a spec of "approx. 10 Kohms" is actually pretty close and "probably" okay. 

This sounds like a great idea - it would address the open-circuit observation in the KS harness for minimal cost/effort. Even if it didn't solve the problem, it would likely clear the 34 code and enable me to look elsewhere.


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## tmoat (Jul 30, 2021)

jp2code said:


> I got my BS in physics back in 1996. It's nice to see someone who knows what an o'scope is.
> 
> If the truck runs, even crappy, with premium, just go for a long drive and burn it off. 300 miles, tops, if yours is like mine.
> 
> ...


My first thought was an ignition problem, because it happened suddenly on the weekend; it was running perfectly up to last week. I can't find any fault in the electrical path, however. The timing belt was replaced a year ago, i.e. about 5000 miles, so it should be fine. I am a little reluctant to run it with this much misfiring as I don't want to blow gaskets or worse. 

We were in school at the same time - I finished my Masters in Electronics Engineering in '97 - I'm a signal processing type, so feel more at home with a spectrum analyzer, but I don't have one at home. When I retire I would like to go back to study Physics and Statistics - you can never know too much of either. 

Thanks again for your suggestions. This will be interesting...


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## tmoat (Jul 30, 2021)

rogoman said:


> Here's several things that could cause your lurching/lugging:
> 
> What brand of spark plugs are you using? You should be using OEM NGK plugs; other brands such as Champion or Bosch many times cause driveability problems in Nissan engines.
> The cam position sensor may be marginal.
> ...



Lots of great information here - thanks! I will respond to each in order: 


I am sure the plugs/wires are NHK - they were last replaced by my local Nissan dealership and they know I want to keep this one on the road.
Dealership technician checked fuel pressure first (his first hunch) and confirmed it was fine
Cam position sensor is worth looking into - noted.
I did a cursory manual inspection of the entire accessible intake system - no fault found. MAF sensor looked clean and I think it works because I was able to generate code 12 when I disconnected it and the code cleared when I reconnected it. Sadly I don't have vacuum tools at home, but I will ask the dealer to do that if I need to take it back to them. Thanks for the clear instructions and expected values - great help!
That's the first mention of fuel injectors, and I think that's possible too, because they haven't been replaced or serviced (I don't think) for the last 25k miles, i.e. since I bought it. I'll try the cleaner.


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## jp2code (Jun 2, 2011)

Do you have a timing light and know how to use one?


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## tmoat (Jul 30, 2021)

No problem with using a timing light, but I haven't owned one since the late '80s. I use the truck only for my renovation work on the weekends. I am repairing what is very likely the last ICE I will ever own - my primary vehicle is a Leaf. I am a little nostalgic about that, but EVs are ever so much more efficient with far fewer moving parts and systems, and time is becoming very precious in my life.


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## BRubble (Jun 22, 2014)

tmoat said:


> Starting a new thread on this topic as it has deviated somewhat from where it was initially posted.
> 
> 1994 VG30E King Cab 4x4 SE V6 truck, VIN 1N6HD16YXRC402885.
> 
> ...



TMoat,

Rogoman Said:


"Dirty fuel injector(s). Run some good injection cleaner, like BG products 44K, through the system; give the cleaner about a week or two to do it's job."
I use Techron "Complete Fuel System Cleaner" every couple year's depending on how the engine is running: 1 oz for 1 gal gas I think is the ratio. If you think there might be some water in the tank then add some Sta-Bil to the tank as well according to the Chevron Tech, but be careful not to add too much of either. I use Sta-Bil in all my Gas that is Stored; Good Stuff IMO. You can get both @ Walmart.

Be sure to put the Additive in First and then add Gas @ the Pump to mix it up good. You don't have to wait until you've used up all the gas that is in the tank right now, unless you've all ready put an additive in.

I would try and do the simple thing's first before doing too much Analyzing. It's hard to tell the difference between the Fuel Side and the Ignition Side. 

If the Truck was running good and all of a sudden started running bad, it might very well be a dirty injector or some water in the system.

Good Luck,


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## tmoat (Jul 30, 2021)

Thanks again to those who offered advice on this and my previous thread - particularly smj999smj. 

It looks like the root cause(s) of the problem was a faulty distributor assembly, possibly damaged by a faulty ignition coil. Now that I see the parts, it all makes sense. The centre contact on the underside of the (not very old) distributor cap was corroded and was probably not making good contact with the rotor - see photo. I suspect this is because the secondary impedance on the ignition coil was too high (measured 12.4kohms vs nominal 10k specification). Further inspection of the distributor assembly revealed a frayed wiring harness, possibly thanks to a small rodent - I hear many people have had rodent problems this year for some reason. None of the wires were open-circuit, but they were frayed and possibly shorting - see second photo. 

















After replacing the distributor assembly, distributor cap, and ignition coil, this old motor is purring like a giant kitten. 

I also opened the wiring harness to the knock sensor as far back as I could, and noted what appears to be a non-factory splice in the ground shield. The splice is solid, and I was unable to find any other anomalies in the wiring. The knock sensor might still be dysfunctional, but it's not causing me grief right now. 

Thanks to all who contributed to this thread - today was a good day. - Trevor


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## BRubble (Jun 22, 2014)

tmoat said:


> Thanks again to those who offered advice on this and my previous thread - particularly smj999smj.
> 
> It looks like the root cause(s) of the problem was a faulty distributor assembly, possibly damaged by a faulty ignition coil. Now that I see the parts, it all makes sense. The centre contact on the underside of the (not very old) distributor cap was corroded and was probably not making good contact with the rotor - see photo. I suspect this is because the secondary impedance on the ignition coil was too high (measured 12.4kohms vs nominal 10k specification). Further inspection of the distributor assembly revealed a frayed wiring harness, possibly thanks to a small rodent - I hear many people have had rodent problems this year for some reason. None of the wires were open-circuit, but they were frayed and possibly shorting - see second photo.
> 
> ...


Trevor,

Glad you got your '94 Hardbody going again.

I've read other post's where SMJ has talked about the Distributor's going bad and causeing problem's.

It's amazing to see these Truck's still going 27 year's after they were manufactured.
Regards,


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## tmoat (Jul 30, 2021)

Hi Barney - I agree. 

These were well-engineered trucks which is why I want to keep mine going. It's getting harder to source parts though. The Nissan service advisor told me he has had three or four similar vintage hardbody 4x4s coming in for major service work, and in all cases the owners are choosing to invest more in repairs than the book value of the vehicles. I want this truck to last the rest of my working life - another ten or fifteen years, maybe 50,000km. It's lasted this long already; with proper care and attention it should last half that long again. 

I really appreciate the advice and experience you and your colleagues provide on this forum - those suggestions helped me keep an otherwise perfectly good truck out of the scrapyard. 

- Trevor


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## BRubble (Jun 22, 2014)

tmoat said:


> Hi Barney - I agree.
> 
> These were well-engineered trucks which is why I want to keep mine going. It's getting harder to source parts though. The Nissan service advisor told me he has had three or four similar vintage hardbody 4x4s coming in for major service work, and in all cases the owners are choosing to invest more in repairs than the book value of the vehicles. I want this truck to last the rest of my working life - another ten or fifteen years, maybe 50,000km. It's lasted this long already; with proper care and attention it should last half that long again.
> 
> ...


Trevor,

Something else that does them in is Wreck's, so for those of us that want to keep them going for a Long Long Time, I personally think that it is good to keep the Collision Insurance in affect and Drive Super Defensively.

I kept my '93 Hardbody for 20 year's and probly would have kept it,, but I needed more pulling power. It was in 2 accident's during that time. One was a Rear End Impact that was aprox 700 $'s sometime around '98. The other was when the Truck was parked @ a kentucky Fried Chicken and someone backed into the Right Front around 2003 that was 1800 $'s; I guess they really gunned their Vehicle while in Reverse and was a Hit and Run. Probly Drunk/Drugged.

The longer we keep them, the harder it is to keep them going, but it can be done cause there are plenty of People Still Driveing much older Car's and Truck's.

I hope my '13 with almost 17K Mile's will be my last, but we just can't foresee the Future.

Best of Luck,


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## smj999smj (Jan 1, 2006)

For awhile, the insulation on the electric wiring of Asian-based vehicles contained soy, which mice will eat. That's the reason rodents like to chew up the electrical harness on these vehicles. Honda later developed "rodent tape" that is infused with pepper and can be wrapped around electrical harness to deter rodents from chewing up the harness; there are aftermarket companies that make it, as well. 








Rodent Tape


Check out the deal on Rodent Tape at College Hills Honda



www.collegehillshonda.com


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## Fourfortheroad (Dec 29, 2021)

tmoat said:


> Starting a new thread on this topic as it has deviated somewhat from where it was initially posted.
> 
> 1994 VG30E King Cab 4x4 SE V6 truck, VIN 1N6HD16YXRC402885.
> 
> ...


I had a similar problem with my Nissan, but at higher RPM's. What I found was a defective or dirty MAF {Mass Air Flow Sensor}


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## rogoman (Dec 16, 2004)

tmoat said:


> Thanks again to those who offered advice on this and my previous thread - particularly smj999smj.
> 
> It looks like the root cause(s) of the problem was a faulty distributor assembly, possibly damaged by a faulty ignition coil. Now that I see the parts, it all makes sense. The centre contact on the underside of the (not very old) distributor cap was corroded and was probably not making good contact with the rotor - see photo. I suspect this is because the secondary impedance on the ignition coil was too high (measured 12.4kohms vs nominal 10k specification). Further inspection of the distributor assembly revealed a frayed wiring harness, possibly thanks to a small rodent - I hear many people have had rodent problems this year for some reason. None of the wires were open-circuit, but they were frayed and possibly shorting - see second photo.
> 
> ...


Glad to see that you got it running.That's what is good about this forum; lots of knowledgeable folks here.


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