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2005 Buick LeSabre stalls while idling

Last November my car began stalling while idling at stop lights. It didn’t happen all the time, just occasionally. The car would start back up without too much difficulty. I took it to a GM dealer who cleaned the throttle body and decarburized the intake and exhaust valves. They also removed an aftermarket security system which had been installed by the previous owner which they claimed may have interfered electrically with the engine. They reported a history of code P0128 (thermostat sticking open) being stored in the ECM. I did not have the thermostat replaced.

The work the dealer performed did not resolve the issue. During the winter, I’ve been letting the car warm-up for a few minutes before driving. While warming up, the rpms will sometimes increase or decrease 200-300 rpm then return to normal idling a few seconds later. Sometimes when the rpms dip down low enough, the engine sputters and either recovers or stalls. On most days I don’t have any issues and the car runs fine. The check engine light sometimes comes on while this happens, and sometimes it doesn’t.

Any advise would be greatly appreciated.

One thing that can account for both a hunting idle and stalling at idle is a faulty idle air control valve (IACV). I would hope that this was cleaned when the throttle body was cleaned, as that is where it is mounted. If not, then it should be cleaned. When it is idling funny after warm up, if you give the IAC a whack or two with a screwdriver handle or something and it affects the idle, this would tell you that it has an issue.