This is a 92 305 TBI and my idle speed is around 1500 RPM. When the motor gets up to temp. it still stays at 1500 even with me pushing the throttle blades fully closed. I have set the base timing to 6 degrees advance, and my vacuum reads about 23 but I think thats because its ideling so high. I played with the idle screw adjuster but that doesn't help any.

What could be causing this condition and what do I need to check?

 

check for a vacuum leak or a hose not connected somewhere, i had this same problem on my S10, i found a open vavuum port. i put a plug there and now it idles where it should.

 

Vacuum leaks will cause high idle. Make sure all the vac lines are hooked up right and if you have removed some make sure they are plugged.

 

Did this just start doing it for no reason or did you buy the car and it was like this? If its a new problem then Id lean towards a vaccuum leak. If its been this way since purchase then Id check the min air setting on the car. To adjust the min air youll need a scan tool. Insert scan tool into aldl, and turn switch on (do not start). Leave it turned on for 30 seconds while the IAC adjusts out. Turn switch off and unplug the IAC. Start the car and let it warm up if needed. Then adjust the min air screw on the front of the tbi near the throttle linkage until your idle is around 750-700 rpms in park. Turn the car off and plug the IAC back up. Restart and check the idle then. This may or may not be your problem, but a lot of ppl dont know this is adj. or even how to do it properly. If youve already attempted this then Ive waisted my breath hehehehe... good luck.

 

ok I just noticed that..... you cant just play with that screw. Its not a carb. Go through the steps I described above to reset the min air where it needs to be and then try finding your problem. Your IAC my also be bad and stuck open. Ive had a brand new one adjust out but not be able to adjust back in.

 

check the gasket between the throttle body and intake.you can check it with carb and choke cleaner.mine idled at almost 2000 till i found it one day.when i fixed it i had to raise the idle.if you do find its the gasket its only 2.99 at autozone.good luck

 

Thank you everybody for the replies. I could have any one of those problems you guys mentioned so let me explain a little more about this car. The car is an 87 Iroc that had a LG4 from the factory. I swapped that out with the TBI motor from my 92 RS. When the engine was in the RS it worked great, not a single problem at all. Now after this swap is when this high idle problem occured. I won't have a chance to mess with the car again until the weekend, but I was pretty sure all the vacuum ports are going someplace or are blocked off. I am also having a problem with the ALDL connector, its not working. I am using the 92 engine harness with the 87 dash harness so I think the pins might not be in the correct places. Oh one last thing, I don't have a VSS hooked up so could that possibly be part of the problem. The car is a stick and I dont have cruise so I didn't bother with it.

Anyway I'll try some of the things you guy mentioned, but if you can think of anything else please let me know.