# high idle when not in gear after TB/IAC/MAF clean 99 sentra gxe manual



## jamesm113 (Jun 20, 2010)

So I cleaned the throttle body and IAC last weekend and now whenever I shift out of gear, or put the clutch in, the idle goes up to 1500 for a few seconds then falls down to the normal idle (800rpm). Not a huge deal, but it's pretty annoying.

I used a can of throttle body cleaner, took the throttle body out of the car and cleaned it off. It was pitch black with carbon deposits, so it obviously needed a cleaning. While I was in there, I took out the throttle plate to better clean it. I also cleaned the MAF with non-chlorinated, residue-free brake cleaner and electric parts cleaner.

Any ideas? Thanks!


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## LvR. (May 26, 2010)

Only when hot?


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## jamesm113 (Jun 20, 2010)

happens when hot and cold.


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## LvR. (May 26, 2010)

IAC and AAC perhaps still/now have dislodged gunk stuck in there somewhere causing them to be slow to respond?

Throttle body not assembled properly? ............... throttle plates not properly located in their previous position? - the plates tend to wear a ring in the throttle body if they are not properly adjusted (they should never physically touch the body) ...............perhaps some "idling jiggling" eventually cause them to fall back/reposition.

I would ensure the throttle plates are adjusted properly and then go about setting the idling on the motor before any further faultfinding since cleaning of the TB is bound to change its flow characteristics anyway.............


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## jamesm113 (Jun 20, 2010)

How would I adjust the throttle plate properly? The action was smooth and the plate wasn't catching on the body.

What's idling jiggling?

Thanks


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## jdg (Aug 27, 2009)

My '98 200SX-SE does exactly the same thing.
*I* think it's got to do with the ECU anticipating a gear change, either up or down, but others might disagree...and the reason I say this is because I've had 2 twins of this same vehicle and they did exactly the same thing.
I cleaned out my pieces/parts also, even changed the IACV, tweaked the minimum air screw, double checked the timing, and a few other things.
Did the same thing with revv'ing up and down, before and after the 'fixes'.
Ya kinda annoying. I ignore it. Everything else runs like a champ.
Might something be misadjusted? Maybe. Might something be a bit out of whack? Sure. Do I care? No...


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## jamesm113 (Jun 20, 2010)

can we do an idle re-learn? or is that for newer sentras?


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## jdg (Aug 27, 2009)

Nope...no idle relearn for us. But you MUST make sure that your minimum idle air screw and timing is set properly for everything else to work right. I've double, no triple/quad checked mine and both of them do the same thing.


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## jamesm113 (Jun 20, 2010)

bummer, i was hoping for a quick fix 

The other thing i noticed was the IAC seemed to default to be slightly open- is that normal? I could move it back and forth with my finger, but it would reset to the slightly open position


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## jdg (Aug 27, 2009)

It's not a real stepper motor as some would think it is. As long as it moves back and forth freely, it should be fine (although one of mine moved freely, it didn't move under ECU control!).

And as long as you can have the engine idling smoothly with the headlights off, A/C off, blower motor off, and so on...
Turn on the headlights/blower motor/step on the brakes, all at the same time.
The engine should maintain the same idle rpm after a quick, and I mean QUICK, blip in the idle speed. If it does, the IACV is working as it should.


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## LvR. (May 26, 2010)

If the TB cleaning caused this I would still blame the resting position of the throttle plates for the issue - I would try to somehow change and reposition the plates' resting position so that there is less of a gap to the TB itself - any gap here will tend to cause higher idling that the AAC/IAC/ECU must compensate for .................. and if there is no tight control on the IACV via a stepper motor, a linear motor can battle to reposition the valve accurately - especially if the default position of that valve seems to be not closed


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## jdg (Aug 27, 2009)

Oh, I agree completely. Maybe the 'coke' in the throttle body was holding the plates up nice and tight against the case...or something like that.
But in this case, I think it's the ECU programming.

I remember reading about this sort of thing about a year ago. The way I remember it is that it's got to do with a chunk of the software called the 'throttle follower logic'.

If the TPS is below, say 10% (just throwing out a number, don't know if it's accurate or not), let's say the TPS is at 5%, the ECU will assume idle rpm is commanded and move the IACV accordingly.
Move the TPS up to 6%, and initially, because you opened the throttle plates, the idle will go up, but the ECU will catch it and close the IACV a bit to get it back to regular idle. Move the TPS up to 7%, same thing happens.
Until you get to 10%, then the ECU quits trying to run the idle, and closes the IACV completely, and the engine runs at whatever rpm the throttle plates let it run at ('cause as far as I can figure, the engine wants all of the air coming thru the throttle plates...I think)
So, here we are tooling down the highway at 20% throttle, going for an upshift. We dump the clutch and release the throttle at the same time. The engine is rev'ing down, and the ECU is anticipating it so it opens up the IACV all the way. The engine stays at 1500 rpm (or whatever) until the ECU catches up and starts to ramp the IACV back down to a normal idle.
That's the way I figure it anyways.
Another thing I think is kinda strange, and basically supports the theory above...
Warm up the engine completely (assuming everything is running correctly), pop the hood, and move the throttle by hand, very slowly. Open it up a very small bit, idle goes up then back down, open it up again a tiny bit, same thing. Normal idle is about 825 rpm. It's almost impossible to hold a steady rpm between about 1000-1200 or so. I can't do it. Maybe somebody else can, but I can't! And that's on both of my GA16DE's. Throttle closed enough and the IACV does the work, throttle open far enough, throttle plates do the work. In between, the ECU jumps back and forth.

Kinda reminds me of the old Q-Jets with rotten throttle shaft bushings. Ya...try and find THAT vacuum leak...

That's my theory anyways. I'm going to see if I can dig that document up that has that whole 'throttle follower logic' theory thing going on with it.

Not specifically for Nissan's, but the reasoning applies:
http://www.diy-efi.org/gmecm/papers/747iac.txt


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## LvR. (May 26, 2010)

"Q"? ............... Quadra?

In that case probably one of the easiest to tune and optimize and make a small-block Chevy purr like a kitten - used to love them and preferred them to Holleys and Webers etc.

Good description of your reasoning and it sure would make sense yes. Lucky me I guess I have not bumped into the dreaded Sentra idle issue, but if I had your hassles I would definitely yank that TB and go to work on modifying it to lessen the resting gap of the butterflies in the throat - I know it will drive me bonkers if I had to live with that sorta nonsense ................. guess it comes from being spoiled by the magnificently controlled idle on my Maxima.


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## jamesm113 (Jun 20, 2010)

I did the IAC test - lights off, no brakes, no blower, and then turned everything on, idle did not budge one bit. So I guess a bad IAC can be ruled out, correct?

I will try the TPS test tonight


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## jdg (Aug 27, 2009)

That's what I would guess, as long as you turned everything on all at the same time, or at least as quickly as possible, within a second or so.
Could you shoot a quick video of the tach and throw it on youtube?
TPS test?


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## jamesm113 (Jun 20, 2010)

Backing out of the driveway in reverse then switching to 1st (high idle starts around 0:04):




Popping out of 3rd:





let me know if I should try again

Ya, I turned everything on at the same time, the RPMs did not change one bit.

Yea, for the "TPS test" I tried applying light throttle to see if it would go back down to idle - it did. Then I applied a bit more, and it fell back down to idle. As long as it the initial revs were under 2k, it would fall back down to idle.


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## LvR. (May 26, 2010)

Is it possible you guys have a leaky/non-seating FICD - if you unplug its connector does anything change in the behavior of the motor?

If you screw the idle adjust screw in totally, is the idle still controlled?

.....................if so, I would try to partially blank off the ports on the AAC so that the engine can only experience a much more limited range of ECU induced throttle bypass due to the AAC operation.

Is it possible to lessen the butterfly seating position in the TB by any means and does this affect the idle control? - in the end you only cleaned stuff and now everything is screwed so there must be an obvious answer related to that cleaning action ............ and imo the TB is the answer

Ultimately it looks to me like the ECU (ROM) somehow needs to be told that the engine parameters changed from the last setup (amount of air allowed through the TB with butterflies closed) .............. and in reality it may be the absolute only way to do the job properly


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## jdg (Aug 27, 2009)

I would think the same thing, about a small vacuum leak around the FICD or IACV or wherever, except that the LT and ST trim values coming from the OBD are practically zero'd out at low rev's, loads, high map, etc...ya know those conditions around idle where a decent vacuum leak would make a difference.

I've actually tried the minimum air screw in all the way. The idle is barely controllable. If I load it down (i.e. A/C, power steering load, headlights, etc), there's only so much that the IACV can handle and the idle dies off a bit, down to about 600rpm if I remember right.
And conversely, when I backed out that screw too much (i.e. basically induced a vacuum leak), the idle takes WAYYYY to long to figure out where it needs to be and jumps around while the ECU figures out what's going on.

I've still got my '98 apart (waiting for the head to get done being re-surfaced). I'll shine a flashlight in thru the throttle bores and see if there is any noticeable light leakage (and therefore air leakage) in there with the throttle plates closed.

Then again, it could just be a stack-up of things... Both my GA16DE's, one has 168K miles, the other 179K miles, heck, they could just be worn out overall far enough that the original ECU programming isn't able to compensate far enough and/or fast enough for all the worn out stuff. Not saying that the engine is worn out, but talking about overall...that whole "tolerance stack-up" thing where individually nothing is shot, but on the whole, something might be out of spec.
I mean really, did Nissan expect these engines to run so good...for so long?


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## jdg (Aug 27, 2009)

jamesm113 said:


> Backing out of the driveway in reverse then switching to 1st (high idle starts around 0:04):
> ..........
> Popping out of 3rd:


Yep, that's exactly what mine does. Doesn't do it each and every time, and it's not anything that sounds destructive or like the ECU is running away with the engine or anything crazy, but that's pretty much it.
I ignore it. I know it's there. I ignore it.
Now if the rpm was hanging up at something like 2,500 rpm...ya, now I'd be worried, but that little 1,500 rpm hang up thing. I think it's due to the ECU programming and the nature of the beast.
If somebody else has a video of another 97-99 manual that DOESN'T do it, I'd like to see it, 'cause I think, while the issue is a bit weird, it's normal-ish.


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## jamesm113 (Jun 20, 2010)

FICD - fast idle control device, correct? where is that located? I only remember seeing the IACV.

What's the LT and ST trim values mean?

I was under the impression that the butterfly valve had only one position inside the TB - with the B stamp on the top in the back, I don't think I can adjust it any, but I will try wiggling it around with the screws loosened. I will also try the flashlight test to check for leakage. 

Then I will try the idle screw test as well.

Also, for what it's worth, my gf (who owns the car) thinks the issue has been going on since she's owned it, although it might have gotten worse after I cleaned it.

Thanks!


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## jdg (Aug 27, 2009)

IACV on the throttle body
FICD / AAC on the passenger side of the intake
LT/ST = Long Term & Short Term fuel trim values that OBD2 uses for instantaneous fuel injection correction based on O2 feedback. 
Other than the rev issue, it's running ok overall right?


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## jamesm113 (Jun 20, 2010)

Other than this issue, the engine runs great. 115k. No CEL or SES light. It's more of an annoyance than anything else. But on the other hand, i want to fix it before it gets worse.

After my TB cleaning (and some tire inflation) the highway gas mileage went up from 33mpg to 37mpg


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## LvR. (May 26, 2010)

Novel idea ................ how about you remove the PCV valve out of the intake equation - remove pipe and block on intake side - leave the sump side open .................... idea being that blow-by (engine wear) is upsetting the whole lot.

Any change?


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## jdg (Aug 27, 2009)

That's not a bad thought...
If there is any blowby, I mean come on...these are Nissan engines! They're tight! They don't have blowby!!!
I'll throw in the thought that excessive blowby could be compensated for by dialing down on the minimum air screw.


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## jamesm113 (Jun 20, 2010)

removing the PCV valve seemed to help a little bit, but the issue was still happening... Didn't get a chance to try the other things last evening.


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## jamesm113 (Jun 20, 2010)

Okay guys a couple more questions

1. I could only find the IACV on the throttle body. Where exactly is the FICD and AAC? Is the AAC the same as the IACV??? Is the FICD the fast idle cam (see question 3)??? this is the 1.6L engine, so i'm not sure if the 2.0L has different components.

2. The throttle plate has the B on Back side, correct? I loosened the screws and did my best to put it in the middle of the throttle body.

3. I tried lowering the idle screw (on the upper part of the throttle body), it had no affect - there was a gap of about .024 inches between the throttle plate assembly (for lack of a better description) and the screw. It was being prevented by the springed part below it (more on this on question 4) Is this normal?










4. Kinda an extension of 3... The factory service manual says i need to measure the stroke of the "thermo-element" (see attached photo) of a part called the fast idle cam... should i do that with the spring attached (see attached photo). There was no gap here.


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## jdg (Aug 27, 2009)

Hold on a sec...
What year/model is your vehicle and engine anyways?


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## jamesm113 (Jun 20, 2010)

99 sentra sedan GXE (1.6L) with manual transmission


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## LvR. (May 26, 2010)

1. You wanna post pic of *your* engine/components then we can point it our if its there

2. Not sure since I never had to deal with the disaster you are chasing - perhaps our buddy JD can help?

3. That is not an idle screw - yes it will/can affect idling but thats not its purpose. The screw is there so as to ensure the butterflies locate properly and repeatedly in such a way as to "*just* not touch" the TB throat body (it prevents the butterflies wearing a ring in the TB) ................ and IMO it needs absolutely to come to rest on the screw else there is something wrong there


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## jdg (Aug 27, 2009)

jamesm113 said:


> 99 sentra sedan GXE (1.6L) with manual transmission


Ok, that's what I thought.
You can NOT adjust the idle without putting the ECU into timing adjust/idle adjust mode. If you're not doing that, you're wasting your time. The ECU is going to combat you at every step trying to maintain whatever idle rpm it wants to.
There's a sticky around here somewhere about setting the timing and MINIMUM AIR screw on the intake. Read them...do them. Has to do with hold the engine rpm at 2-3K rpm for a minute, blipping the throttle, shutting it off, unplugging the TPS, starting the engine, and a few other things. I can't think of the exact sequence at the moment.
Or get yourself a Haynes or Chilton's manual and follow the instructions in there.

And X2 on what L.V.R. said above...

IACV = idle air control valve (drives the idle rpm)
AAC = auxiliary air control (adds extra air when the IACV can't handle the whole air load)
FICD = fast idle control device (same thing as the AAC)

The IACV is on the same end as the throttle plates. Quit messing with the throttle plates unless you know there is a problem. Hopefully you put some locktight on those screws if you loosened them up, otherwise you'll end up with metal in the engine eventually.
The AAC/FICD is mounted on the other end of the intake from the throttle body.

Update: After looking thru a few of the FSM's for the various years, it looks like on some years, the IACV and AAC/FICD are on the same end, and in other years they are on opposite ends.


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## jdg (Aug 27, 2009)

I think I see the problem.
The FSM you're probably looking at is for a '96. You've got a '99 (Update:maybe). A few things changed for the '97-99 models.
Not to mention there are a few differences between the USA models and Canadian models, specifically in the locations and operation/functions of various emission components.
http://www.aubreyandcharles.com/ServiceManuals/
Did you download the manual from this site?
As far as I can tell, it's all good for the Sentra/200SX types, correct years and that sort of thing.
Personally, I think you're chasing a non-existent problem and/or a problem that is just due to the age of the vehicle, normal wear and tear and so on.
But, maybe there really is an issue...so I guess, keep pressing on...


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## jamesm113 (Jun 20, 2010)

engine pics:

































throttle plate with B:


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## jdg (Aug 27, 2009)

jamesm113 said:


>


Well, no wonder it ain't running right! There's no throttle body on there!


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## jamesm113 (Jun 20, 2010)

hahaha, yea, i was wondering why it was running so rough! 

Yes, I had the 96 FSM... I just checked the FSM for the 99 sentra GA engine. the IACV-AAC and IACV-FICD are all together on the TB for me. I'm glad that's all cleared up now...

I think the fast idle cam might be out of spec - the thermo-element stroke is measuring 14mm (with the springs on) and 15mm (springs off) in about 60F, which is on the high end on the chart either way. Think that could be the cause of this issue?

Thanks!!


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## jamesm113 (Jun 20, 2010)

jdgrotte-
I found the sticky you mentioned on the GA16DE forum (link), but i couldn't find anything in the thread about the minimum air screw - they showed how to set the idle, but i don't believe i have that screw, just the minimum air screw, which is the upper screw, correct?

Thanks


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## jdg (Aug 27, 2009)

Yes, the minimum air screw, which is the 'base idle', kinda like setting the 'base timing'.
As I might've mentioned before, the IACV can only add air, it can't subtract it.
So, you set the base idle, to something like 500 rpm or whatever the book want. The real idle will be at something like 800 rpm because the IACV can ADD air to the idle.
The minimum air screw is the one under the rubber plug on top of the intake, almost right nexto the the throttle body. The screw itself looks like a big ol' #3 phillips head screw.


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## jamesm113 (Jun 20, 2010)

Well, I'd given up on this (probably the better way to go) but it started bugging me again, so I'm back to work on it...

Tonight, I took out the fast idle cam, and removed the min air screw. These steps helped improve the condition, but it still existed. However, the throttle plate stopper wouldn't go flush with the throttle body - it seemed the throttle plate got stuck on the inside of the throttle body - is that normal?? - I spent half the night trying to get the throttle plate to line up perfectly and did the best I could, but there was still a gap of about 2-3mm


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## LvR. (May 26, 2010)

jamesm113 said:


> Well, I'd given up on this (probably the better way to go) but it started bugging me again, so I'm back to work on it...
> 
> Tonight, I took out the fast idle cam, and removed the min air screw. These steps helped improve the condition, but it still existed. However, the throttle plate stopper wouldn't go flush with the throttle body - it seemed the throttle plate got stuck on the inside of the throttle body - is that normal?? - I spent half the night trying to get the throttle plate to line up perfectly and did the best I could, but there was still a gap of about 2-3mm


IMO and experience its not normal and not desirable either - that valve should have a few "" of clearance in the TB only and it must always come to rest on the throttle stop screw (that effectively and repeatably defines how much play there is between the TB body and the valve) - the idle control is supposed to happen external/independently to/from the TB ................ what you have now is the idle control system having a much smaller effect on the idle speed and its thus struggling to both learn and control the idle speed.

There must be some reason why the valve in the TB doesn't close off the TB better - typically I have found in the past that the valve for some reason got mis-located on the shaft - ie - its either too far to the one side


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## jamesm113 (Jun 20, 2010)

Yea, the problem is I spent almost all night trying to get it aligned perfectly ...and couldn't There was still some gaps, so perhaps i don't have it oriented correctly. Ugh... A used TB is $13 on ebay right now... tempting...


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## jdg (Aug 27, 2009)

Jeeze, I'm almost thinking you've got it in upside-down or something, 'cause something just ain't right with that. I looked at the throttle body on my '98. Shined a flashlight thru the front, looked at it from the back. I can barely see the slightest sliver of light between the throttle plate and the throttle body itself. The sliver of light is practically all the way around the plate, but it's there, and again, VERY small sliver.
That $13 for a TB might not be a bad deal.


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## jamesm113 (Jun 20, 2010)

jdgrotte said:


> Jeeze, I'm almost thinking you've got it in upside-down or something, 'cause something just ain't right with that. I looked at the throttle body on my '98. Shined a flashlight thru the front, looked at it from the back. I can barely see the slightest sliver of light between the throttle plate and the throttle body itself. The sliver of light is practically all the way around the plate, but it's there, and again, VERY small sliver.
> That $13 for a TB might not be a bad deal.


where is the B on yours?:
http://i1028.photobucket.com/albums/y347/jamesm113/DSCN3494.jpg


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