Issue is that when in Park, and I hit the gas pedal the rpm goes up as expected, but when I release the gas pedal the rpm's stays up for 2 to 3 seconds before it slowly goes back to normal idle.
I already replaced the throttle body, gas pedal, and purge valve, but issue remains.

There are no codes.
Any idea what else I need to look at?

 

Hello,
I am having the same issue. Cleaned old Throttle Body, Then replaced it with a new part. Was looking for the IAC valve but I can't t find it anywhere and suspect the ECM uses the Throttle to accomplish the same thing.
Did you find a resolution?

 

Eventually I brought my car to the shop and told them that I replaced the throttle body and gas pedal (which has a sensor too) with new ACDelco parts, but the issue is still there.
I even tried the 'relearn' procedure but that didn't fix it either.

So they calibrated the new throttle body and gas pedal with the ECU and all is working fine now. Cost was $165 which was not bad in my opinion.