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Q: The car engine runs erratically when it is cold, then runs just fine after the car warms up after a while.

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Been trying to figure out the cause for this. Spark plugs, fuel filters, o2 sensors...replaced quite a few parts but the problem persists. In the morning after the car has cooled overnight, it runs rough. It sputters, chokes, hesitates, the RPM dances up and down when I accelerate.. it's just not smooth. After a few miles of this the car finally warms up to what I guess is operating temperature and the ride smooths out completely. The problem dissapears. Until the car cools down again. Any idea what it might be that's dependent on the engine temperature? It hasn't done this before at all; surely it being an old car has to be a factor. Thanks!

My car has 171000 miles.
My car has an automatic transmission.

Try checking the coolant temperature sensor. When the engine is cold, the enrichment function relies heavily on the coolant temperature sensor to determine how much fuel to supply at lower temperatures. If this is not working properly, it may struggle to settle into a well running pattern. For example, if it is 30 degrees outside and the temperature sensor reads 100 degrees, this may send false information to the ECU causing it to send false information to the fuel injection system. A qualified professional from YourMechanic can inspect the rough running problem and verify the needed repairs.

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