Wandering Idle

courtesy of Bob, The Auto Answer Man

Hi Bob,
I have a 2000 Mustang GT and just got it back from the dealer after they worked on it for two weeks. They replaced the IAC valve, but I noticed it was idling erratically with the A/C on yesterday. I didn't notice it while driving. However, when stopped at lights, the engine would drop from the normal 800 RPMs to about 500 and then surge to 1200 when it started to stall. Any idea what the problem is?

It's possible that either:

1) There is a vacuum leak.
2) The map sensor is bad.
3) The IAC is bad.
4) They did not give the car a sufficient learning cycle to learn how to operate the new IAC.
Good Luck!
Drive Safely!
Bob, The Auto Answer Man

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