IAC maybe?

[scotts62]

I have a '92 350/4l60 Xcab and it seems to have a higher idle after its warmed up and driving, it seems to come off high idle a bit as it sits and warms up but when i go and drive it and stop at a light or something its idling higher but as im sitting there it slowly starts to drop and idle where its suppose to, im just wondering if its the IAC or something else. Any ideas?

[ChevyTech]

Try stopping vacuum on the hose going to the brake booster.

If the booster gets a torn diaphragm in it, excessive vacuum will be flowing, and it creates a vacuum leak.

We did a lot of diagnostics on one of the boards recently and the problem was the booster.

Check for other vacuum leaks too.

An engine coolant sensor failing or with a bad connection can make the ECM think the engine is cold and the ECM will speed up the RPM.

The IAC could be getting bad.

Sometimes when the distributor gets worn, the shaft wobbles around, and the ECM gets a poor RPM signal for controlling the idle speed.

How’s that for a few ideas!

[scotts62]

Originally Posted by ChevyTech

Try stopping vacuum on the hose going to the brake booster.

If the booster gets a torn diaphragm in it, excessive vacuum will be flowing, and it creates a vacuum leak.

We did a lot of diagnostics on one of the boards recently and the problem was the booster.

Check for other vacuum leaks too.

An engine coolant sensor failing or with a bad connection can make the ECM think the engine is cold and the ECM will speed up the RPM.

The IAC could be getting bad.

Sometimes when the distributor gets worn, the shaft wobbles around, and the ECM gets a poor RPM signal for controlling the idle speed.

How’s that for a few ideas!

You always seem to always have a few ideas that help!

I think i can rule out the distributor because it is new (dont mean anything though) the coolant sensor is new also but you know how the new parts go sometimes..

I will try the booster line and see.

Funny thing was is this morning after i posted the question and went to leave for the day it warmed up fine and drove good and even at stop lights all was good so it got my hopes up but then coming home it was doing it again

[Boog]

If the spring is broken in the booster it can puncture the diaphram and plug the hole sometimes as well as it moves back and forth so it leaks sometimes and not sometimes.

[bwood]

Could it be a cracked line between the EGR Solenoid and EGR valve creating a vacuum leak? I was thinking the EGR didn't begin to operate unit the truck warms up.

[scotts62]

Havent had a chance to check anything out, caught the funk and have been down the last day or so but will try all your ideas asap. thanks guys

[Mechanic77]

Just another suggestion.... the throttle body base gaskets on those have a tendency to get sucked in, causing a vacuum leak and a high idle. The gaskets cheap, and it only takes 15 min. to change it I've seen probably a dozen or so in my 10+ years of wrenching.

[scotts62]

Well right after i asked the question i got Pneumonia and that kept me down and indoors for a while but im back now. I pulled the booster line and held my thumb over it and it didnt change RPM at all so i think that rules it out, right?

I have a spare newly used cool temp sender that im going to put in and see what happens.

[ChevyTech]

Yes; that rules out the booster if it still runs to fast with your thumb plugging the hose.

When checking for leaks make sure you check the base gasket under the TBI. As Mechanic77 stated it is a common problem.

You said it eventually slows down, so the IAC could be getting bad a sticking.

Usually the IAC does not go bad electrically, but it is common for them to stick and stop moving.

The IAC is a stepper motor and uses short pulses of electricity on the wires to move its position.

If the IAC sticks as does not move as it should for each pulse, the IAC count will get off because the computer does not know the actual position of the IAC, but the ECM tracks the commands it has sent to the IAC and assumes it has moved every time it has been sent a pulse.

To clean the IAC, you should remove it, and use a wet rag to wipe the tip of the plunger, while holding it with the plunger pointing DOWN. Be careful, the IAC will break easily. It is aluminum. Clean the passages in the TBI unit while you have the IAC removed.

I do not suggest unscrewing the pintle (plunger). If you don’t get in back in far enough, the IAC can get damaged if the “pintle” (plunger) bottoms out in the TBI, when you screw the IAC back in.

You can look down from the top and see the IAC plunger if it is in the normal idle position.

Glad you are feeling better.

[scotts62]

Originally Posted by ChevyTech

Yes; that rules out the booster if it still runs to fast with your thumb plugging the hose.

When checking for leaks make sure you check the base gasket under the TBI. As Mechanic77 stated it is a common problem.

You said it eventually slows down, so the IAC could be getting bad a sticking.

Usually the IAC does not go bad electrically, but it is common for them to stick and stop moving.

The IAC is a stepper motor and uses short pulses of electricity on the wires to move its position.

If the IAC sticks as does not move as it should for each pulse, the IAC count will get off because the computer does not know the actual position of the IAC, but the ECM tracks the commands it has sent to the IAC and assumes it has moved every time it has been sent a pulse.

To clean the IAC, you should remove it, and use a wet rag to wipe the tip of the plunger, while holding it with the plunger pointing DOWN. Be careful, the IAC will break easily. It is aluminum. Clean the passages in the TBI unit while you have the IAC removed.

I do not suggest unscrewing the pintle (plunger). If you don’t get in back in far enough, the IAC can get damaged if the “pintle” (plunger) bottoms out in the TBI, when you screw the IAC back in.

You can look down from the top and see the IAC plunger if it is in the normal idle position.

Glad you are feeling better.

Thanks, ya me too that was bad

Well i rebuilt the engine about 10k ago and put a complete new gasket kit in, should i pull the TBI and just check? I wouldnt think it would be the gasket but ya never know..

Just drove it to the hardware store and when i come up to a light and stop it idles about 900 (if i were to put it in park it idles up around 1500) but slowly starts to idle down to around 300 (ish) about 30 seconds after i stop.

Ok i will take it out and clean it up and get back to you with what i have found.

[ChevyTech]

If the TBI base gasket is only 10,000 miles old, it is probably good.

When you did the engine work did you spray the TBI unit off with carburetor cleaner? If you did that could have damaged the IAC or the TPS. If the ECM does not see an idle value on the TPS voltage, then the ECM will not be trying to control the idle speed.

[scotts62]

It just started acting weird the last week or so, i just noticed that the idle seemed a bit higher than normal. I know that doesnt mean that i couldnt have messed it up though.

I took all the sensors out and cleaned just the aluminum body and put it back together, if i remember correctly it wasnt all that dirty but that was about 8 moths ago and a lot has gone on and i rebuilt the engine for my crew cab so i may be a bit confused

[scotts62]

Well i pulled the truck in the shop today and changed the oil and a couple other things and while i had it in there i took the IAC out and compared it to the one i had from another truck and this is what i found..

Original one is on the right, new ish is on the left


Put the new one it and it runs a lot better!

[Boog]

Of the few IAC's I have put on they each had instructions with them saying measure the amount the pintle sticks out. I believe the starter measurement was 1 1/8" on my last one. Once installed turning the key to the on position for a few seconds will let it seat the rest of the way. It said over extending the pintle before installing it can damage the unit. My 2000 developed an idle problem and I was able to remove it, clean it and reinstall to correct it. Been good for 7 or 8 months now.

[scotts62]

I had someone turn the key as i watched it and it didnt move at all, hope thats all it was.

[ChevyTech]

I think you have found the problem and it may run even better after you drive it.

When you replace, remove and cleaned an IAC, or unplugged the IAC with the key on, the ECM may need to relearn the IAC position. This will not happen without driving the truck and meeting specific conditions or taking other steps the cause it to relearn the idle.

Re: Boog’s comments
If the pintle (plunger) is extended while off the TBI unit and it is not retracted before reinstalling IAC will get damaged if the “pintle” (plunger) bottoms out in the TBI when you screw the IAC back in.

The tip of the pintle must not extend more than 28mm which is 1.10 inches from the shoulder toward the electrical connector, by the threads.

I had someone turn the key as i watched it and it didnt move at all, hope thats all it was.

It should have moved.

After the engine is shut off, and before it is started, the IAC should open somewhat. When the engine is cranked, and if the IAC does not open at all the engine will usually “load up” or “flood”.

Being yours was stuck open that is why it did not flood or load up, but it should have moved.

[scotts62]

Drove it for a while today and it seemed to run a lot better so i think its good to go until the next problem

Thanks for your help!!

[scotts62]

Well it ran fine for a day or two and its back to the high idle.. going to swap the cool/temp tomorrow and see what happens.

[Boog]

Scott the cts don't cost much anyway but Chevtech has posted many times the info to test them if you choose to do so. While you are there clean those ground wires up real good too. Couldn't hurt.

[scotts62]

I have a new one to throw in it but may test it just to see what i get.

[scotts62]

Changed out the cool temp and all seems to be back to normal for now, thanks guys!

[ChevyTech]

It has been two days, is it still running ok this time?

[scotts62]

yep thanks for thinking of me. I went out and warmed it up a few mins ago to check the antifreeze before our possible winter storm in the next few days and it was running just fine

[Camaro_nut2001]

Originally Posted by ChevyTech

After the engine is shut off, and before it is started, the IAC should open somewhat. When the engine is cranked, and if the IAC does not open at all the engine will usually “load up” or “flood”.

Being yours was stuck open that is why it did not flood or load up, but it should have moved.

I just put a new pressure regulator diaphram in my '89 & since the kit came with all the gaskets & o-rings I went ahead & cleaned everything & installed all of the new o-rings, filters & gaskets. Well, it cured my hard startin when cold but now after it warms up & I shut it off for a few min., when I try to start it back up it's flooded. Do you think I damaged the IAC or maybe have a leaky injector now? It has never done this before. Thanks & I don't mean to hi-jack this fellers thread.

[ChevyTech]

Originally Posted by Camaro_nut2001

I just put a new pressure regulator diaphram in my '89 & since the kit came with all the gaskets & o-rings I went ahead & cleaned everything & installed all of the new o-rings, filters & gaskets. Well, it cured my hard startin when cold but now after it warms up & I shut it off for a few min., when I try to start it back up it's flooded. Do you think I damaged the IAC or maybe have a leaky injector now? It has never done this before. Thanks & I don't mean to hi-jack this fellers thread.

Immediately after you shut off the engine look at the TBI unit and see if fuel is leaking.

A damaged injector o-ring can allow fuel to dip in the throttle bore. A gasket leak can on the upper TBI also cause this problem.

Another problem is that the TBI unit is thin by the injector o-rings, and if lube is not used on the o-rings, sometimes the TBI aluminum cracks in the area of the o-ring.

Another possibility is the coolant temp sensor connector is in poor condition and got bumped when you were working under the hood. Any resistance or connection problem will make it read colder then it really is. Colder means more gas gets injected.