I have a 1989 corvette with a zz4 block. I recently changed the cam to a lt4 hotcam, long tube headers, edelbrock high flow intake manifold, slp runners, ported plenum. I seem to have a issue at idle only when the car is warm. If the car sits at idle for about 30 seconds, the rpms drop to 400 and then rev up to 900, then it will stall out. I have proper pressure at the rails, new bosch III 24# injectors, and a new 255lph fuel pump. The car is running the stock tune. TPS reads .54v at idle. It does not start doing this until the car is at about 170+ degrees. Any ideas?

 

Any change is going to required the base idle to be set for the ecm expects that to be correct and cannot maintain a stable idle if something has not been set right. You are having the problems when the car is going from open loop when cold to closed loop when at normal temp .

first thing to do is disconnect and re-connect your battery as the ecm will detect faults and make adjustments to try and rectify them. ie retard timing.

Set your car to a base idle (when hot) at normal operating temperature.

Connect a paper clip, to your diagnostic terminal under your dash connect to terminal A and B (the top right two pins).
Turn on ignition but don't run the engine.
wait at least 30 seconds and then remove the idle air valve connector (IAC) (the square plug) then turn of ignition and unplug the paperclip.
Now remove the timing connector a single wire near the fuel pump relay.
Start your vette and set the idle speed to 400 rpm in neutral (auto trans) 450 rpm (manual trans) then check your throttle position sensor,
the top two wires it should be set to 0.54volts dc, also confirm with engine off that the tps rises to 4.5 volts at full throttle.
Turn of the ignition and reconnect the iac valve connector and timing connecter and start your vette.
it should go to 1200 rpm then drop slowly to 600 rpm, take the car for a 15 minute drive with the usual stop lights.
It is normal for a little hunting while the ecm relearns
All should settle after 15 to 30 mins normal driving and have a stable idle.

Good luck

 

I will give that a try tomorrow. I am going to also check for vacuum leaks.