This IAC is really starting to anger me. The car is a pain in the *** to drive, start, shift, everything because of this crap IAC. The procedure for "resetting" the IAC doesn't do anything either. Can somebody explain how the "IAC reset" does anything useful?

The IAC by the nature of it can not give feedback to the PCM as to it's actual position. The computer just sees that more air is needed and bumps the steps up not knowing whether or not it really is opening. Now what I need to know is say I scan my car while it's off and the counts read 130. Technically shouldn't there be a constant measurement for how far the pintle sticks out of the IAC body depending on how many steps it is open. Like for example, 130 IAC steps is the same as have only 12mm of the pintle coming out of the IAC. I want to start with a new IAC and in the exact position that it needs to be in. How do I do that?

 

The IAC reset is meant to take care of the synch. When you hold the throttle open and start the engine, it should force the IAC to bottom out (or nearly bottom out), because it does not need to allow much or any air in.

It's normal for the counts to be high when you look at them with the key on and engine not started. That seems to be the initial starting point until the engine actually fires up and not an indication of trouble. Extremely high counts while running would be a cause for investigation.

 

Thanks for the info shoebox.

So if I get this right, by holding the throttle open the IAC motor bottoms out and closes the air passageway or nearly closes it and at the same time, the PCM is bottoming out the IAC steps to compensate so they both synchronize near the bottom of the range?

I see... So how long do you usually have to keep your foot cracking the gas pedal a little bit after the car starts? Is it like once it fires up you instantaneously release the gas pedal, or is it more like fire it up with the gas open and hold it for a couple seconds then let go, or is it more like fire it up with the gas open and hold for the full five seconds and then shut it off??? Maybe I'm not doing it right...

 

Depress accelerator slightly
Start engine, then release accelerator pedal, run engine for 5 seconds
Turn ignition "OFF" for ten seconds
Restart engine and check for proper idle operation

You keep your foot on the pedal until the engine fires or maybe a half second longer. You don't want to it revving too high at startup.


As you noted, you just have to trust that the movement of the pintle corresponds with the counts the PCM is commanding.

Have you cleaned the IAC passages?

You have not really said what you car is doing, other than it is a pain to drive.

 

Well first off every time the car starts up no matter what it revs to between 1500 and 2000 rpm and takes a good 10 seconds to return to normal idle. It takes off the line crappy, like the transition from no throttle to part throttle is rough. The revs do not come down fast enough between shifts so it makes the shifts kinda of a pain to make it smooth.

Tell me this though, if you reset the IAC, the computer will only move the pintle to the point where it is bottoming out. Once you reach that point it's not going to keep trying to make the motor open. For that reset idea to work, the IAC motor has to move a different amount of steps (positive or negative) than the IAC counts in the PCM are moving so that they can be recalibrated. If you somehow had the PCM and the motor go out of sync the only way to get it back in sync is to physically move the motor if my thinking is correct. What do you think about that??? The only thing I'm not sure of is whether or not the PCM stops sending the signal to lower the step count to the motor once the PCM bottoms out (or does it even bottom out, I have never seen it completely close)...

 

I can't tell you that the IAC reset procedure is infallible. It's the way the factory manual says to reset it. If the IAC and computer get out of synch (like if you pulled the IAC connector off when the engine was running), the reset procedure is supposed to take care of it.

 

Isn't there also a specification for how far the pintle is supposed to project from the IAC motor on a "new" uninstalled unit? Would a pintle that has been forced or damaged possibly cause a problem like this?

Since the engine isn't running smoothly, I ssupect its not the IAC that's causing the problem, but if you want to rule is out, might be worth checking the pintle.

 

Fred, the reason the car is not driving smoothly is because the revs don't return down after shifts, every time I start the car it revs up past 1500 and takes a long while to come back down. I feel very strongly that the IAC is the problem for various reasons. I may just be the motor itself however I tried another IAC from a running car and it seemed to not provide enough air. The thing that gets me is that the idle takes such a long time to come down because the PCM is counting down so damn slow. I scanned the car at startup and the counts are creeping like a snail back down to about 50 or 60 counts starting out at 130 (which is respondible for making the car rev up at startup).

Say for example you have a car with the computer reading 0 counts but you know for a fact the IAC is not close all the way. Will the computer keep trying to close the IAC when it reaches zero counts or will it just assume that the IAC is closed and stop trying?

 

If you have a low vacuum motor, try opening the throttle blades out a little, not enough to raise the idle, and see if this helps. ai just went through this about an hour ago, and because of my cam, 230/236 its vacuum is a little low causing a bad idle, especially with air on. SO the IAC takes over as it is trying to get air in the motor, and it maxes out, open, so when I opened the throttle blades to get the IAC counts down to about 50, it idles fine and everything else is much better as far as driving. Hope this helps.

 

My cam is on a 115 lsa... makes about 15" of vacuum warmed up which is plenty.

 

What if the PCM doesn't see the true engine rpm?

 

This part doesn't sound like an IAC problem to me. I could only imagine an IAC that's completely closed causing a bog if you gun it quickly but that even sounds a bit goofy as I read it back to myself and you'd be complaining of too low an idle if that were true.

Your other problems sound like possible IAC problems, but not the above, unless maybe it's so bad it's thrown off your fuel counts at idle.

Have you checked for vacuum leaks?

 

The engine RPM is impossible to miscalculate. It's dependant on physical movement (cps, or opti spark optical wheel, etc.) and inherently has feedback mechanism so the PCM knows the real status of the RPM. Say the computer did not know the exact position of the RPM's, that value is tied into so many other functions that the car would not run right at all.

 

I agree, that could be timing or something else like that. However, the IAC is making shifting smoothly pretty difficult because it wants to hold the rpms up when I shift, so when the clutch engages it pulls the engine down instead of them being almost equal. It's like it's trying to let in too much air.