1987 5.0 Iroc Z. Recently rebuilt engine. Brand new IAC, have a OBD1 computer for the car

Was able to find the breakdown on how to adjust the idle and it worked. I was happy as can be. Before, car was running at 1700 rpm in park and neutral and 900 in drive and reverse.

Turned off car, let it cool, turned it back on and rpm went back up. Put computer in, turned ignition to on position, unplugged IAC, turned car to off, plugged IAC back, removed computer, started car and rpm worked great.

Then when I shut the car off and then back on, my RPM goes right back up again.

Also, if I plug the computer in and start the car, it starts much better and runs with the lower RPM.

Any suggestions?

 

You might have a vacuum leak since the motor was out recently, double check your connections, check vacuum line connection under the throttle body.

 

If it was a vacuum issue, I do not think I could get it to run fine 1 minute, turn it off and idle jumps the next.

Because of this and the hard starting, I am going to replace the ECM, fuel filter and fuel pump.