I need some help with my old 86 IROC, which is now my brother's car. The car has just recently started refusing to idle below 1750-2000ish rpm.

I've checked the codes, there are none, and I've reset the IAC a couple times as per the instructions in the tech section. Which is where it gets weird. Immediately after resetting the IAC, the first time you crank it the car it runs great. I drove it for like 20 minutes and it ran perfect, idle normal. I shut it off and restart it and then it's back to 2000rpm again. I've reset it several times, it's the same each time. First time recranking after resetting the IAC it idles properly until shut off and restarted. Then it returns to high-idle-land.

I thought perhaps the IAC was just getting stuck, but while it's wanting to idle so high if I manually try to bring the idle down with the throttle set-screw it drops some, but then the computer somehow sees that and jerks the idle speed back up, so it would appear the IAC is not sticking, the computer is telling it to stay that high. So now I'm drawing a blank. I don't have a digital volt meter to check the TPS, but would it having jumped out of spec cause the idle to act like that? How come after resetting the IAC everything works great until I shut the engine off and restart it? If it's the TPS wouldn't it stay high regardless?

Any help or suggestions would be appreciated. I'm pulling the IAC out to clean the thing just to eliminate it sticking 100%, but I'm thinking it some other sensor that is responsible (but it's not throwing any codes...).

Thanks ahead of time for any ideas or suggestions...

 

Yup, I've got the same problem only not as bad. Upon startup it goes to 1500, after about 15 Sec it settles down to 800/900. I've done all the recommended fixes, IAC, TPS (.54V), 18 inches of vacuum and rock steady, no codes...got me baffled ..I'll be watch the threads to see what you can dig up..Good Luck

 

Cleaned up the IAC, no change. Anyone got any ideas?

Probably going to have to pay some shop to hook it up to a diagnostic setup or something, I've got nothing else to check that I know of short of buying a DMM and checking the TPS (probably do that tomorrow)...

 

Replace the IAC. There's more than 1 coil in that thing. that's why it seems to work in certian ranges. it's only llke 50 bucks.

 

Ray,

Whip out your trusty ohmmeter and check the resistance of the CTS. It may be drifting away....

The values should be close to this: