My daughter's '89 Cadillac makes a thumping sound as the key is turned to start the car or just after the key is turned off. The sound comes from the ISC motor pushing the throttle 3 or 4 times which also makes the gas pedal bounce. This started several months ago soon after I had a fast idle problem. I replaced the oxygen sensor and the ISC motor and had a shop adjust the ISC motor. The idle speed is good and I had tried disconnecting the battery and letting the computer relearn the idle.

Thanks.

Your shop may have not correctly adjusted the idle speed on the Caddy. The ISC will do that if it is not correctly adjusted. Search for "idle speed adjustment" and look for the responses by bbob, and readjust the idle speed your self.