87 Iroc, 305 TPI. When driving the car for awhile, when I come to a stop with my foot on the brake, the car will creep to a faster idle. It will usually go from around 700 to 900. Car runs fine and doesn't throw any codes.

Also if I shut the car off and engine temp is still hot, when I restart it, it acts like its cold. Engine rpm goes to around 1300 and then comes down after about 30 secs. Does this sound like a typical IAC problem?

Thanks as always!

 

You could use a DMM to check the voltage across the IAC at idle to get an idea if it is the cause. Vacuum leaks and fuel pressure fluctuations could also be a cause of what you describe. Your idle rising for about 30 seconds before returning to normal, that sounds like your ECM is compensating to correct the issue. I would start with a vacuum guage after looking for any visible vacuum leaks.

 

Thanks for the suggestions. I was actually checking the IAC and found it finger tight with a home made gasket on it. I went ahead and replaced it. Pulled throttle body apart and throughly cleaned. Not much better. The car does have a Hypertech chip in it.

Were are you suggesting to hook up vac gauge? What is a typical reading for this engine at idle?

Thanks

 

A normal reading depends on your engine config. When I had mine, I installed an aftermarket vacuum gauge where my A/C controls used to be so I could always monitor it. A stock reading should be somewhere close to 20 inches. As to where to hook up a hand gauge, thats up to you. You can tee into a vacuum line or unplug a line from your TBI unit (not the one going to the MAP sensor) and plug in there. I recommend the one going to your cruise control.

It is good you found the problems with the IAC. If it was finger tight with a homemade gasket, it is a strong change that it may be the source of your rough idle. Use a DMM to see how it behaves while the engine is running on and off idle. Is your engine light on?

 

Didn't throw any codes. Actually the engine idles fine, not rough at all. Just creeps up. I know the SES is working as it threw a VSS code a month ago when my speedo cable broke. I took it for another ride today after the IAC replace and it still creeps up a couple hundred rpm's. Would a TPS have any inpact on this? On the IAC, what voltage am I looking for across what pins at idle.

Fuel pressure looks good and stable

Thanks again for your help...

 

I dont have the wiring diagram at this moment but I will post an answer to that question later once I dig up my manual. Pretty much you want to see if the IAC is actually functioning the way it is supposed to, and if the ECM is intentionally raising your idle speed. From what you have said so far, I strongly believe your problem is some type of vacuum leak. Try to get a vacuum reading asap. You should have a reading between 18-20 inches at idle, the higher the better but definitely not lower.