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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.

A: When an engine is cold, it is usually runni...

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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