1991 RS Idle Problem

My 91RS TBI has an idle problem. When I put the car in gear, or engage the A/C the RPM's drop down and the engine eventually dies. It almost feels like it is missing. I have cleaned the IAC and replaced it with a new one and neither solution fixed the problem. Anybody have any idea's on what is wrong?

Does the a/c clutch freewheel with no drag?

I will check the A/C clutch....thanks. I do want to add that the engine acts like it's pulsing...the rpm's can go up and down anywhere from 400 to 800 rpms.

Please list anything else out the norm. no matter how insignificant it may seem.

My reccomendation would be to check very thoroughly for any vacuum leaks.

Vacuum leaks (even very small ones) can cause very irregular and inconsistent idle speeds on TBI. The ECM will become confused and have no idea what to do with the IAC so it either becomes more or less inoperative or will run in and out causing a fast then slow then fast idle speed since it cant find a "set" point

Have you checked/reset the throttle minimum air position RPM? Is the PCV valve the correct part for the engine (that can make a significant difference)? What is the normal idle RPM? BTW - Don't rely on the dash "tachometer" for RPM indication, since they tend to error high most of the time - Use a handheld. That thing in the dash is more to fill an empty space than to accurately indicate RPM.

I checked the TPS voltage today by turning the key on but not the engine, disconnecting the plug on the TPS. My voltage read 4.91?

You need to backprobe the TPS when it is connected to get a reading.