Pontiac Repair: Idle hunting..., 2002 pontiac sunfire, pontiac sunfire

Question
2002 pontiac sunfire se, 2.2L, automatic. wife first noticed that the car had a "really rough idle".  after some investigation, i determined that it happened only in drive when the a/c is on.  after more looking i realized i could make it stop if i stopped the car really slowly.  it seems that when you stop harder the engine speed drops to around 500 rpm.  after that the controls try to compensate and rpms overshoot and undershoot continuously.  if i turn off the a/c it will stop but when i turn it back on rpms jump up (as i believe they are suppose to in expectation of the load) and the hunting starts all over.  i replaced the battery a few months ago and i think thats when this started.  ive tried disconnecting and reconnecting hoping the car would "learn" how to idle, but the problem just isnt correcting itself.  finally, my question is, are there any adjustable settings in the controller that were cleared when the battery was replaced and need resetting?  is there any manual adjustment that sets the 'baseline' idle speed or is there a way to adjust whatever the dead band is to prevent the hunting?

Answer
Hi
I would first check for ANY vaccuum leaks. The PCM (computer) MAY be trying to compenstae for a vacuum leak and is not able to do so. Could be either an external or internal leak.
I would also recommend having the Throttle body cleaned also called a Fuel Induction service.(can be preformed at MOST repair shops)
It will clean carbon deposits from around the bore and the IAC (Idle Air Control) passages.
Another "trick" to try would be to unplug BOTH the positive and negative cables from the battery.
After they BOTH UNPLUGGED, touch them together for 30 secs to a minute.
Then hook the positive back on first, then the negeative.
This will clear up any "Logic lock" that MAY be creating your problem.
If these two items do not resolve your condition, I would either just replace the IAC valve, or take it to a good repair shop and have the idle system diagnosed, which they can do by commanding the iac valve in and out and checking for the proper signals from the PCM (computer) and making sure it is doing what the PCM is commanding.
I Hope this helps.
JUST advice from experience.
Good luck
Pawl