92 Cutlass Supreme S rough idle when warm

Greetings - I have experienced this problem 6-7 times, and a new idle/air/control valve has always fixed the problem. Not this time. I have read many forums, it appears this is a very common problem. Unfortunately the answer seems to be elusive. Tried the coolant temp sensor with no luck. Next I can try the O2 sensor, the EGR (which is only 2 yrs old), the intake gasket was replaced 2 yrs ago, so I doubt it is a vac leak there. Also the mass airflow sensor, or the coil pack, or the fuel pump regulator, or the crank sensor, or bad injectors? I don't know where to begin. It seems many people have tried all of these things and still have problems. The car idles up in neutral to 2000 then drops to 500 loping. Rich mixture. It seems the computer is sending out bad info to the fuel injection system because when the car is run during the first 10 minutes, everything is perfect. Good idle, smooth. Then when it reaches 200 degrees something changes and it runs like crap. Please help! thank you :runaround: ps - I'm honestly thinking the new IAC is bad. Can it be tested by autozone?

3.1 or 3.4 L?
Try this link for testing the IAC.
http://www.gearhead-efi.com/Fuel-Injection/showthread.php?273-IAC-Testing-procedure

Thank you for that link ^ but I don't understand electrical nearly that well. I was wondering if the people at autozone check their own parts for us. The car is the 3.4 engine. Would it be beneficial to pay to have the codes read? Will it give info on specific sensors or just a general overview of the problem? T/Y sparky

If the cel is on, yes get the codes.

Have you tested or replaced the PCV valve lately? I too have a 3.4 Liter cutlass ( a 1993 convertible) and was having rough idle on cold start but it would go away after warm up. I replaced the idle air control and the Throttle position sensor and it worked fine for a while. The car spent many days at a mechancs shop and we were about to replace the fuel injectors when someone noted the pcv valve was not even on the car and the car was runnng very rough. your symptoms sound like either the plenum needs a good cleaning, a new pcv valve is needed, or a tune up is needed?

could it still be a bad temp coolant sensor? I know you said that was replaced but wonder if it is by chance a bad sensor?

I am having the opposite issue (when cold, it intermittently idles rough and stalls). I am going to try replacing the temp coolant sensor and just see what happens. i have also learned on this car nearly anytime you replace a sensor, unplug the battery for about an hour or so and replug it so the computer resets itself.