IAC maybe?
 

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?

Re: IAC maybe?
 

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!

Re: IAC maybe?
 

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 :lol:

Re: IAC maybe?
 

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.

Re: IAC maybe?
 

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.

Re: IAC maybe?
 

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

Re: IAC maybe?
 

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.

Re: IAC maybe?
 

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.

Re: IAC maybe?
 

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.

Re: IAC maybe?
 

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.

Re: IAC maybe?
 

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.

Re: IAC maybe?
 

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 :lol:

Re: IAC maybe?
 

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
http://i276.photobucket.com/albums/k...ruck/045-1.jpg

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

Re: IAC maybe?
 

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.

Re: IAC maybe?
 

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

Re: IAC maybe?
 

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.

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.

Re: IAC maybe?
 

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 :lol:

Thanks for your help!!

Re: IAC maybe?
 

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.

Re: IAC maybe?
 

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.

Re: IAC maybe?
 

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

Re: IAC maybe?
 

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

Re: IAC maybe?
 

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

Re: IAC maybe?
 

:lol: 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 :)

Re: IAC maybe?
 

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.

Re: IAC maybe?
 

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.

Re: IAC maybe?
 

Thank you ChevyTech. I did pull the breather & watched it after turnin the key off & sure enough, the top o-ring on the left side injector was leaking. I pulled the top back off, injector out, found an o-ring in my parts bin that was almost the correct match, enough so that it worked lol, lubed it & the bottom o-ring up with a little wheel bearing grease & problem solved. Thanks for your very quick reply to help me.

Re: IAC maybe?
 

You’re welcome, and thank you for letting us know what fixed it.

Re: IAC maybe?
 

Sorry to revive a long dead thread, but this upcoming weekend not only am I replacing the door hing pins for the second time this year, because the Dorman ones fell apart and the door is worse now than before I fixed it, but I'm trying to diagnose and fix a high idle, bad gas mileage, and sometimes stumbling and hesitation issue in my 92 3500 with a 5.7. I get, and can find an AC delco IAC valve if that turns out to be it, and I'll check the brake booster too, but the coolant sensor in the intake, is it the same one as the one for the cluster? just a second one? auto parts store only shows one for the truck. guage doesn't work either, and gonna check for correct thermostat as well this weekend just because.

Re: IAC maybe?
 

The coolant sensor in the cylinder head for the gauge is different then the coolant sensor by the thermostat that the computer monitors.
The sensor by the thermostat has two terminals. The sensor in the head has one terminal and a prong in the center so if installed by the thermostat the wiring will not plug onto it.

If you want to test the sensor that the computer monitors here is information on that.

To test the coolant sensor, unplug the wire connector and test the resistance across the two terminals of the SENSOR with an ohm meter.

If you see any coolant leaking out the terminals replace it.

If the terminals in the connector are corroded replace the “pigtail”.

On the TBI trucks the sensor screws into the intake manifold, on the front passenger side, by the thermostat housing.
On the Vortec big block it's on the front of the intake ahead of the thermostat housing.

Coolant sensor approximate resistance specifications:


177 ohms @ 212 deg. F. or 100 deg. C.
241 ohms @ 194 deg. F. or 90 deg. C.
332 ohms @ 176 deg. F. or 80 deg. C.
467 ohms @ 158 deg. F. or 70 deg. C.
667 ohms @ 140 deg. F. or 60 deg. C.
973 ohms @ 122 deg. F. or 50 deg. C.
1188 ohms @ 113 deg. F. or 45 deg. C.
1459 ohms @ 104 deg. F. or 40 deg. C.
1802 ohms @ 95 deg. F. or 35 deg. C.
2238 ohms @ 86 deg. F. or 30 deg. C.
2796 ohms @ 77 deg. F. or 25 deg. C.
3520 ohms @ 68 deg. F. or 20 deg. C.
4450 ohms @ 59 deg. F. or 15 deg. C.
5670 ohms @ 50 deg. F. or 10 deg. C.
7280 ohms @ 41 deg. F. or 5 deg. C.
9420 ohms @ 32 deg. F. or 0 deg. C.
12300 ohms @ 23 deg. F. or -5 deg. C.
16180 ohms @ 14 deg. F. or -10 deg. C.
21450 ohms @ 5 deg. F. or -15 deg. C.
28680 ohms @ -4 deg. F. or -20 deg. C.
52700 ohms @ -22 deg. F. or -30 deg. C.
100700 ohms @ -40 deg. F. or - 40 deg. C.

Re: IAC maybe?
 

awesome, thank you. I will check that this weekend.

Re: IAC maybe?
 

While fixing the power steering hose yesterday, and replacing the o rings on the other hose that didn't blow out, I also replaced the pigtail on the intake coolant sensor since it was melted looking. Cranked the truck and used a thermometer to check and confirm actual temperatures, and found that the thermostat was stuck open. Replaced that and now it runs at proper temperature and idles much better. There is still a very intermittent miss but I can hear a sound when it misses so I'm 99% sure that's a plug wire. I will replace those this weekend. Thank you again for all the help.

Re: IAC maybe?
 

Good progress!
If plug wires are not the problem you may consider carbon build up because it was running rich. If they run cold the system is running rich.

These TBI trucks can get very heavy carbon deposits on the valves when they run cold.

I have used SEA FOAM several times with great success. I have also used GMs TOP ENGINE CLEANER at the dealership many times with good results. The GM Top Engine Cleaner smells just like SEA FOAM to me and I have wondered if they are the exact same product. I have never had any problems from using these products, but if an engine has very heavy carbon, one treatment may not get it all out. My biggest concern is that it is not poured in so fast that it liquid locks the engine.

I slowly pour in a half can in the hot engine at fast idle then stall the engine and let it sit ten minutes. Then I slowly pour in the rest of the can at a fast idle, and then go drive it.

Re: IAC maybe?
 

Changed the plugs, wires, distributor cap, rotor, and fixed a couple grounds and some other wires that looked like they'd seen better days as well. Also broke out the timing light and followed the proper procedure of unplugging the tan/black wire to check the timing. it was off a good bit, so reset it to 0 and plugged the wire back in and revved it and all and rechecked.
The plugs looked surprisingly good, driving it a week either cleaned the carbon and rich looking plugs up, or by some miracle with it running as crappy as it was 2 weeks ago the plugs stayed ok.
It runs sooo much better now, no stumbling like when I back off the gas heading to a red light and it turns green and get back on throttle, etc. it's like a new truck again now. Thank you again for the tips and help. :metal:

Re: IAC maybe?
 

You're welcome!
Glad to hear it is running well.
The plugs clean up quickly if it is driven at high enough RPMs when fully warmed up.

Re: IAC maybe?
 

OK all the work I've done lately has made the truck run fantastic most of the time, thank you again ChevyTech. Maybe I should start a new thread but i'm getting replies in this one. One issue the truck has sporadically had, when it sat alot it was more common, since Christmas when it became my daily driver again I almost thought it had gone away, but this morning it did it again.
Driving to work I hit a pothole and the truck went from running at the RPMs needed to keep 35 mph, to idle or less, no throttle response at all, but oil pressure was up, engine was running enough (pretty sure) then it died and check engine light flashed and i was coasting.. shifted to neutral and recranked it and it ran kinda rough but ran for a few more miles. Then same thing again, cut off at 60 mph, but not like flat out died, just no throttle response. Engine died finally and recranked it again and been ok since,

I'm thinking ICM maybe? that's one of the few parts I haven't changed yet. Maybe all the extra heat from having a working thermostat and truck running at 195 degrees is making the sporadic problem more common?
It is my only transportation now days, and starting a new job next week, letting the problem get bad enough to finally fully fail so i can easily find it is a last resort lol.
Thoughts?

Re: IAC maybe?
 

The module in the distributor is a common failure part but that does not fit the hitting a pothole scenario. These do get problems with the connection where the wiring plugs onto the module on the outside of the distributor which does fit with hitting bumps.

First thing that comes to my mind is the jolt affecting the fuel pump. It is common for the hose between the fuel pump sender and the fuel pump to get mushy and leak. The jolt could be affecting that hose. These hoses seem to fail from a vehicle that sits a long time without being driven.

The ignition switch on the lower area of the steering column can get worn and lose connection from hitting bumps.

It would be a good idea to check for trouble codes which can be done without a scan tool or code reader. Here is a link on checking codes.
https://www.chevytalk.org/fusionbb/s...php?tid/273660

Re: IAC maybe?
 

yes from reading other threads you've helped in on these issues that thought has also crossed my mind about the fuel hose deal.. and fuel filter hsn't been replaced likely since last time I personally did it at least a decade ago lol.. (was a company truck but I always felt sentimental to it, since i helped start the company and sold my car to buy it)
As to that fuel hose breaking in the tank, does it matter that it had a full tank, as in minutes before just filled it to the brim to test to see if I now get miles per gallon instead of gallons per mile?

Re: IAC maybe?
 

In past discussions on fuel hose failure some people think the hose fails faster when there is very little fuel in the tank so it is not submersed.

Where I live there is high ethanol content in the fuel. I have seen the fuel damage small engine rubber components so fast I could watch the damage happening.

If your truck had never been exposed to ethanol and had a low quality piece of hose installed during a previous repair and the hose was deteriorating then you put an high ethanol blend of fuel in the tank it may have done damage that quickly.

Re: IAC maybe?
 

Usually it's the factory connector hose that turns to mush. All the replacement pumps I've bought came with a decent hose to reconnect the pump.

Re: IAC maybe?
 

Posting a little more info to possibly help in diagnosing, as well as explanation of history a bit

It did it again this morning. For the last couple years it's only been sporadic, but the engine was running ridiculously cold for the last couple years too. I used to maintain the truck myself but I left the company for 5 years and over that time is when all the issues started.

As I said, it was pretty common at one time, but a new distributor was put in because of the slop in the shaft, and that made it very sporadic again. I've been daily driving since basically new years day this year because the company shut down and the truck became mine. In that time, first 3 months of this year, it's only cut off like that maybe twice the whole time.
Now that the bad thermostat was replaced and the engine is running at 195 degrees again (2 weeks) it's done it now 3 times in the last 2 days, so I'm thinking wahtever it is is heat related. No pothole this morning but shut off again.