Just had problems with idle on my 1991 with only 56k miles on.

Car would start normally with rpm at 1300 but after first gas pedal push it would fall to 500. Things came back to normal after it start to warm up.

I took out the IAC and cleaned it with carb cleaner and blew air to dry it. It does open with 12 volts and seem to move freely. Did not seem to improve anything not to say it works less well than before clean up.

I removed and cleaned the throttle body and checked that the throttle position switch is working as by the green book. That is fine and the continuity is good on the middle and one side connector. The other side does not seem to have continuity with any throttle opening.

I replaced the IAC with an old one I had on hand which seem to work as I just started the car.

The IAC I took out has a strange 1/16 inch gap at the swing valve when closed. In fact it is not closed as I can blow air through with my mouth. It seems the rest seat is worn and the gate moves past the closed position.

Question:
Can I re-adjust the sealed allen screw on the side of the IAC to move the gate properly closed ?