Sounds like the Idle Air Control is sticking. You can try removing it and cleaning it out with carb cleaner to see if it will fix it, or at least make a difference to let you know we are on the right track.It should look like this

Answered by 2CarPros Jack (expert)
12,033 answers provided

I took the IACV out, sprayed it with brake / carb cleaner, shook it a bit, let it dry out, reinstalled it, and the problem persists. The car acts normally (aside from shifting a little high, around 2500/3000RPM), but as soon as I take my foot off the gas, when it drops down to idle, it drops down more and more every time.

Response from crowley1027
2 questions asked

If it helps, the car was sitting for a year or two in a driveway. Had a nice moldy smell and a rat (or squirrel) nest in the back speaker, as well as underneath the under-hood fuse box. I took the head off, and had an apprentice mechanic put it back on to replace a head gasket. It went into a garage where the Coolant sensor was fixed, along with the tie rod end...
Figured I would provide a little information.

Response from crowley1027
2 questions asked

If the ignition timing, cam/crank timing are correrct, the IAC is what controls the idle.They can stick and not set any code, especially if the vehicle sat for a long time. Just for laughs, try tapping it gently while the car is running to see if it makes a change, i have had them move when tapped before, but just die down like you describe

Answered by 2CarPros Jack (expert)
12,033 answers provided

So assuming that there are no vacuum leaks, and the IAC valve is working properly, what would you recommend next?

Response from crowley1027
2 questions asked

Clean out the throttle bore around the butterfly to be sure the correct amount of air is getting in.Use carb cleaner and a rag

Answered by 2CarPros Jack (expert)
12,033 answers provided