I have a 1992 trans Am 5.0 TPI Automatic. I drive it daily and travel about 30 miles to work each day. By time i get to work the idle is very high, around 1000 to 1200. The car is mostly stock and i recently changed the IAC motor about 6 months ago. The only time the idle runs high is after a long trip on the highway. It never does it when i do short trips. I noticed it will do it no matter what the temp of the car is, it could be running cool or warm when in bumper to bumper traffic. Its always right after i get off the highway.

 

Does it eventually go back down or stay that high?

 

it stays that high and doesnt go down but im only off the interstate for about 10 mins or so by time i get to a parking spot at work or my driveway at home

 

Mmk, my car is not tpi (it's carb'd) but after a long drive it will idle high for about 30 seconds or so but then go down.

Maybe something in your throttle position sensor is being weird?

 

I have a very similar problem that's intermittent. It doesn't have to be a long drive but occasionally when I go to pull into a parking spot I feel the car pulling with my foot on the brakes and when I put it in park the motor jumps up to 2000rpm and stays there, I'll shut it off and start it back up and everything will be fine. the car has a new tps and iac both properly set.

I'd verify you're getting correct volts at your tps sensor, should be .56 with the throttle closed and over 4 volts WOT