Q: Car runs badly when it's warm

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When car isn't warmed up yet, runs fine. After being driven for a while and warmed up, when its sitting at idle bogs in and out, sometimes stalling, and sometimes trouble starting.

My car has 135000 miles.
My car has an automatic transmission.

When an engine is cold, it is usually running in a rich fuel state. The engine control unit relies on input from many different sensors, such as the engine/coolant temperature sensors. Once the engine warms up, the ECU readjusts the fuel mixture to a leaner state. Running well when cold then poorly when warmed up can be caused by a number of different things. If there is a vacuum leak, the engine can be drawing in too much air. This will not be as noticeable with a cold engine because of the extra fuel, but when warmed up and the amount of fuel is reduced, things like a low idle, poor performance, stalling, and in some cases depending on the size of the vacuum leak, a very high idle.

Things like the air intake hose, vacuum lines, and the intake manifold / gasket should be checked for leaks. Electronics can also cause this situation even if the check engine light has not come on. A poor performing coolant temperature sensor for example, can sort of confuse the ECU if it is not sending a proper signal for engine temperature. A poorly performing mass air flow sensor can also send poor readings. Have a certified technician look into why the vehicle is stalling, coolant temperature sensor, and the mass air flow sensor to isolate the exact cause.

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