Here's a situation I had while driving. It's a 91 5.0 L 305 c.i.speed density with a stick.
After about driving for 20 minutes on hwy and city, I came to a traffic light and as I was waiting for the green light the rpm climbed suddenly to 2,500. Pulled over and the sound of a major vacuum leak was embarrassing. So back home, I chased the leak and found the IAC was wide open. Stuck my finger in the IAC hole of the throttle body and the rpm dropped to almost normal. Finger too fat to cover the opening. I did the procedure to set the idle rpm and proved the IAC was working correctly because while the ALDL was jumper-ed, the idle was normal. Then I checked the TPS and it read 4.35v all the time from 0 % - 100 % of throttle blade opening. Replaced with another TPS and it read normal from 0.53v to 4.5v. Removed jumper and started the engine and all was back to normal.
What I want to know is if there's someone that can explain why the TPS makes that much of a change. Just knowledge for me and others.
Thank you in advance.

 

It is called 'throttle follower' (TF). The ECM opens the IAC as the throttle is opened. Then when the throttle is reduced or closed the IAC steps down to the proper position. This helps in two ways, reduced HC emissions and helps prevent an engine stall.

RBob.

 

RBob - Thank you for clarifying that. It makes sense now. As always, it's my privilege to have you on this forum. Your knowledge is endless.

 

ok... so can you drive the car with the IAC disconnected?...just set the base to say 800 rpm?...

The car keep starting at 1750 rpm and runs up to 2000 rpm when warm... disconnect the IAC and i can get it back down to something reasonable.... tried changing the IAC with one i had... no change... thinking it may be the ECM....

 

Well, if you read my post that's what mine did except higher rpm's. So my suggestion is to do the TPS replacement routine.