It has a new IAC. It was installed correctly yet the idle is still too high. If I am lucky it will Idle at 1500. It was idling earlier today at 1800 and withing driving half a mile it was at 2000. Drove home(about 20 miles) and then the idle was at 2200. I pulled it in the garage, diconnected the battery and pulled the IAC back out. I measured the distance from base to tip of pintle and it was at 1 1/8 inch. I decided to try and set it out a bit further. I set it just past 1 1/4 inch, put it in, hooked up battery and started it. It idled just over 700 rpm and the check engine, abs, and trac lights were on and I went for a quick drive. Drove just over a mile to the seven eleven. Shut it off while I ran if for a pepsi, can out fired up and was idling at 2000 but lights were off. What could the problem really be? I checked all vacuum hosesand they are good. My throttle body was really dirty and i cleaned it out. Bad TPS, throttle body, IAC(was the same way with the old one), or??? whats it take to have the ecm reflashed? I did pull the codes a couple of weeks ago and it showed TPS and also a lean O2 sensor. I havedealt with bad O2s before but it seems that they have all idled low and ran bad which is not the problem here.

 

Mine wasn't quite that bad, but would hover around 1400rpm. Turned out to be a combination of a worn Throttle Body shaft and intake manifold leak. Does the idle settle down if you blip the throttle? At least a little bit? If it does, the blades of your TB are binding on the walls, and you either need to have the TB bored out and bushings installed, or just get a new TB. I bought one bored to 52mm and re-bushed on eBay for $170.

As for the intake manifold leak, I used butane to find it. Make sure the car is outside, and use a butane torch (not lit, obviously), to spray butane into suspect areas. If the idle jumps when you spray it somewhere, there's a vacuum leak there.

 

Make sure your TB is hitting the stop @ idle too.

 

For future reference:
The 1 1/8" measurement is the distance required to safely install the IAC without mashing the pintle into seat. The ECM controls the IAC and there is no adjustment.

 

when the throttle is blipped the idle does not lower. I will check the manifold for leaks.

 

You might want to run through the "Base idle pricedure" (I believe it is in the Tech Tips section if you are unfamiliar) and see where the base idle is sitting.

Shortly after I got my 86', I cleaned the throttle body. After that the car would idle at 11-1200RPM in gear and 14-1500 in neutral. Resetting the base idle fixed that issue.

 

It ended up being what I suspected, TPS. I have never has a bad one act that way. I changed it and it idles perfectly now.

 

What is TPS?

 

Throttle Position Sensor

 

I had the same problem recently and turned out to be combination of alot of common problems C4s have. Check the Engine Coolant Temperature sensor located under the TB, You'll need a scan tool to do it correctly. If this sensor is bad the engine will run in a "open loop", the ECM thinks the coolant is cold and kicks the ilde way up, ECM runs off stored maps until sensor information shows a warmed up engine. It also sounds like the IAC isn't adjust within it correct window, plus you need to set the idle adjustment screw on the TB, procedures for all the above are on the web.

 

In case you did not read above the problem was the TPS and there is no idle adjustment screw on an LT engine because the computer controls the idle.

 

I was going to tell you to check the TPS reading but you beat me to it with the fix

I've had a ticket to that movie before when I was much, much younger and wound up breaking my IAC trying to adjust it, buying a new one then finding out it was the TPS

Glad you found it.

 

Sorry, thought you had a L98, my C4 is an 1988 and has all the adjustable settings.

 

You are lucky! That was one thing I really liked about my 89