Buick/Olds: general engine, throttle position sensor, vacuum leaks

Question
i have a 1989 cutlass supreme international.the car had a 2.8l mpfi in it but it blew.we replaced it with a 3.1L mpfi..kept 2.8 wire harness cuz 3.1 harness wouldnt fit. got it all hooked up...the exhaust isnt hooked up..the manifold is on..just no exhaust off the manifold.it has trouble starting..which i cant see why with it being mpfi and also the main problem..it revs all by it self. doesnt take the pedal away..just revs..1st u have to keep it running by stepping on the gas every 5 secs then in a min it will just take off and rev to the max..dont know whats wrong need help..thanks
                              Rick

Answer
Well first of all, the computer is programmed for the 2.8 not the 3.1
The reving problem could be a bad Idle air control valve(IAC). Also the computer needs to "learn" the idle speed. The only good way to get it to learn the idle is wit ha scan tool. You need to make sure the minimum air rate and throttle position sensor(TPS) are set correctly and then let the scan tool force the computer to relearn the idle speed.
Make sure that there are no vacuum leaks.