Intermittent high idle

[rabrew1953]

First of all thanks to everyone here who helps us DIY'ers. My 2000 LS 3800II has been revving high RPM's at start up and during idling at city stop lights, signs, and so forth. Not all the time. 50% of the time she will return to normal idle but don't see a coorelation when the engine is hot or cold. I replaced the IAC and that helped a little. I don't see a vacuum leak and the pintle on the EGR is fine. I cleaned the throttle body plate when I saw carbon build up. She has just over 250K on her and I want to keep her running. Any ideas? Thanks for you help.

[GTP Dad]

It sounds like you may have an lower intake gasket problem. Check around the manifold for a vacuum leak. If the gaskets are going bad you will get a high idle condition and it will be hard to track down. Also check all the vacuum hoses, the plastic ones can crack and open up periodically especially when the car is hot. Just a couple of things to check!

[BNaylor]

Did you get all the carbon out of the orifice/seat where the IAC mounts to the throttle body?

Does it make any difference whether the A/C is on or off. What rpms does it idle at when acting up?

[rabrew1953]

Thanks for the help guys. I haven't been able to check all your suggestions. I will tell you this....she hasn't acted up in the last 4 days. Go figure. I was in an accident last November and the the electrical system has acted funny ever since but like the high idle none of it has been a consistent problem.

When she has idled high, she has been around 2k. Haven't been running the A/C that much lately so don't know if it's related.

[BNaylor]

Where was the accident damage?

[rabrew1953]

Caved in the rt front headlamp, slight buckle of the hood and qtr panel. She was pushed in up to and including the battery.

[BNaylor]

Have you double checked the grounding. Ground at inner fender and from battery negative terminal to the main ground located at the engine/tranny grounding stud. Also, inspect the engine compartment fuse box and wiring harness since it is located by the battery and impact area.