I'm working on a 1991 Formula 5.7 TPI engine. The car has high miles but runs pretty good, it has a little bit of a high idle and sometimes stalls going into gear. It also has a hesitiation occaisionally and won't rev over 2,000 RPMs when I'm driving on the highway. I bought a new TPS and IAC valve and installed them. The car won't idle below 1700 RPM with the new parts installed. I put the old IAC valve back in and it still idled at 1700 RPM. I put the old TPS in and it ran like before, slightly high idle etc... What'd I do wrong?

 

Are the throttle plates closing fully? Any vacuum leaks?

 

I think I'm good regarding vacuum leaks and throttle plates. I just don't understand why a new TPS made the idle so high. Is it possible to fix by manually lowering the idle with the screw?

 

Was the tps holes for mounting elongated on new tps ? Voltage on blue wire when installed?
sounds like worn throttle body shaft into body of throttle body.

 

I have to check voltage on wire. Holes were not elongated, I beleive it was the correct, non ajustable TPS. Old TPS re installed, it runs pretty good, new one installed it idled way too high.

 

That is exactly the reason for the inquiry about the throttle plates being fully closed. There is a specific procedure for setting throttle minimum air position, and not just a random adjustment of the stop screw.

However, none of that is relevant until the KOEO voltage measurement of the TPS output is provided.

 

My 91 ran decent but overall but suddenly started running very rich. Would stall out and barely ran. Ended up having several original injectors go bad around the same time. Replaced them and it was night and day different. Check your tailpipes, if they're all carboned up, look at your injectors further. The bad ones ohm'd pretty close to spec, but they were stuck open.