IAC powered with ignition off, 89 PA

This is my first post to a forum, so please be patient. I have a 1989 Park Avenue with 69,000 miles on it. It's in great shape, but an IAC/ECM issue is getting under my skin. Often after a drive it appears the ECM continues to pulse/power the IAC after the ignition is turned off. The pulse duration is roughly 1 second at a rate of every 2-3 seconds. Sometimes this IAC pulsing will go on for only a few minutes after the ignition is off, but often it will pulse for an hour or more (one time all night before I realized it). There are no other obvious symptoms, other than the pulsing IAC. Sometimes I will let it continue until the pulsing eventually stops, other times I will disconnect and reconnect the battery terminal to stop it. Pulling the ECM fuse has no affect. I have not attempted to pull codes yet, but that is probably my next step. I am technically capable, but don't have schematics or enough information on the system to know where to start looking. I assume the ECM is being powered with the ignition off and the ECM is thinking the engine is still running.

Any help with this issue would be greatly appreciated. Thanks!