Skip the auto shop - Our mechanics make house calls

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

asked by on

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.

A: Try checking the coolant temperature sensor...

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.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Car not going over 20 MPH

Hello. If the car will not go over 20 MPH then there can be a number of issues going on. If the Check Engine Light is on then the computer needs to be scanned for codes first. If there are...

Q: Car hesitation from stop to acceleration

Hi, thanks for writing in. Prior to further diagnostics you should install the spark plugs that are referred to in the factory service manual and/or obtain the plugs from a GM dealer. Installing alternative aftermarket plugs with different gaps, materials,...

Q: RPMs erratic during idling, Check Engine Light on, P0174 code

Hello. The P0174 code deals with the truck running lean. That means that the PCM is restricting fuel to the injectors. This is usually caused by a vacuum leak in the engine. There could be a leaking intake gasket, or...

Related articles

How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
P0052 OBD-II Trouble Code: HO2S Heater Control Circuit High (Bank 2 Sensor 1)
P0052 code definition HO2S Heater Control Circuit High (Bank 2 Sensor 1) What the P0052 code means This code is seen when the Engine Control Module (ECM) tries to control the...
How to Avoid Back Pain in a Car
If you have back problems, sitting in a car for an extended period of time can be excruciating. Even without back problems, you could experience discomfort and soreness from...