98 GT high idle


New Member
Sep 23, 2018
Miami, FL
I have a 98 GT with 128k miles.

Recently I stopped at a red light and all of a sudden the idle went from around 750 to about 1800. It stayed there constantly and only dipped into 1200 when the AC compressor cycled, but then again went to 180 and sometimes even to 2k. The idle goes back to normal once and I turn off the car and immediately start it back on. The high idle sometimes comes bac after about 10 min and sometimes it idles perfectly the entire time I am driving it. What could it possibly be? There is no CEL either.

Have not looked into it much but I know that the engine dies once I unplug the IAC. Could it be a vacuum leak or TPS? Maybe the TB?

There is no effect on driving apart from the fact that I can let go of the throttle and the car will keep driving with no RPM drop (think of a cruise control being on). Does that mean I have a constant metered air coming in even if I don't have the food on the gas?

Any advice would be appreciated.
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