I need some help! I am really scratching my head. I recently bought a 90 Trans Am TPI with no engine. It had been setting for 9 years. I rebuilt a stock roller cam 305. Bored .030 over and everything else bone stock. When I first got it running, everything was OK. It would idle about 700 rpm. After I put a couple hunderd miles on it, the idle speed starts going high 1500-2000 rpm's. I had a "mechanic" scan the ECM. He said that the IAC was bad. So I replaced it. About $95. Still high idle. Any suggestions before I start throwing more money at it?? I have tried resetting the IAC three different ways. The instructions with the new IAC says to run the car about 40 MPH. Didn't work. I tried putting the ALDL in diagnostic mode by jumping A & B. Didn't work. I tried looking in a factory service manual. It say to start the car at part trottle and let it rum for 5 seconds then shut off. Didn't work. I need help please.
Thanks,
Andy

 

The most effective method is the ALDL method. Enter diagnostic mode with the A nd B connectors jumpered, KOEO. Wait about 20 seconds to fully extend the IAC pintle, disconnect the electrical connector from the IAC, then turn off the ignition. Use a handheld diagnostic tachometer to check and set the minimum air RPM. The dash mounted tach is hardly accurate, and usually reads high.

If you cannot adjust the throttle minimum air RPM to the specification with this method, you have other problems. A sticking/binding throttle plate, vacuum leaks, damaged IAC, IAC pintle, or valve seat in the TB are possibilities. Vacuum leaks can include gaskets, hoses, the EGR, PCV system, power brake booster, injector 'O' rings, damaged castings, or damaged EVAP system.

 

Update. I found out the TPS and MAP were both bad. After I repaced those, it has ran fine.