Well my 91 S-10 with 4.3 TBI is acting up on me, I put a new IAC in and even changed the connector going to the IAC and its still idling way to high. I can reset the IAC and start the motor and run around and the idle will start out normal and gradually get higher and higher. I have no tach but if I had to guess I would say the idle is between 2500 - 3000. If I am in gravel and put it in drive it will spin the back tire thats how bad it is. I really dont know where to start on this one. I am running with the IAC unhooked and idle manually set which is alright till I try to run the AC or cold starts. What should I try next??

 

It sounds to me that the throttle body gasket is leaking and that is why you have the high idle.I have seen this many times.I hope this helps.

 

I will pick up a gasket this weekend and see what that does. I noticed another weird thing when i put the timing light on it when I hit the gas the timing retards for an instant then starts advancing like normal, I have never noticed that before, might be timing chain, motor has 225,000 miles on it.

Thanks

 

That is normal, when you crack the throttle the MAP increases which changes the lookup cell, retards the timing, then as the rpm comes up the timing advances.

I am going to use the attached SA table to show you why.

Lets say that your engine idles at 600 RPM and at 50 KPA. The timing advance from the lookup cell is 27* (for simplicity I am throwing out all of the adders, etc). Now open the throttle, the engine is still turning 600 RPM due to inertia, but the MAP increases to 80 KPA which gives 13* advance. Now hold the throttle for 1 second, the RPM has risen to 2,400 and the MAP has decreased to 40 KPA as the engine picks up revs and is now restricted by the partially open throttle plates. The timing now reads 33*.

 

im having the same problem with the tpi though. i put on a bigger TB and have a mild cam in it and i hear a slight whistle from the TB with slight acceleration. my idle doesnt seem to get higher until the warms up. do u think it could be the same problem?