I have a 99 Seville STS with 80k miles on it and it suddenly started idling high and isn't giving any codes. It seems to happen after the car is warmed up and I don't notice it (obviously) until I start to slow down and need extra pressure on the brakes and you feel it "clunk" down in to first gear. Once the car is in park it wants to idle above 2000 rpms. I replaced the IAC Valve myself but to no avail. The dealer looked at it and said it was most likely a vacuum problem and cleaned the "flapper" valve (as he called it) on the intake plenum and charged me $100....No luck! It has always made a high pitched sound at that valve and never caused any idle problems before so I doubt that is it. I have read other posts but haven't found anything useful. Any help would be greatly appreciated!! Thanks!

 

Well it seems to be a leak somewhere. Get your ear ready for some investigating. I would tell you what to pray on what parts for finding leaksbut I totally forgot what I used when I was back in school. But I am sure others will chime in on this problem.

Check the postion of the throttle postion sensor too.

 

Thanks to everyone who viewed my post and replied. It turned out to be the Throttle Position Sensor. I replaced the IAC and when that didn't work and there was no vacuum leak I replaced the TPS and have had no problems in the last week. This seems to be a common problem for a lot of people on here so make sure and replace the TPS even if it seems to be fine, $40 is a small price to cure a headache!

 

Well here isa couplepart numbers forthe TPSfor those whom have this problem and want it easier when looking at part stores.

Delphi - SS10382
or
Borg Warner - EC3048

There are more part numbers out thereobviously, but have the numbers cross referenced brand to brand if possible if you choose this route.