Skip the auto shop - Our mechanics make house calls

Q: Car runs badly when it's warm

asked by on

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.

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

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: Why won't my car move in reverse. 2004 Honda Accord

Hello. From what you are explaining this is the first sign of a serious transmission failure. This is a common issue on this vehicle. The Honda Accord has a history of transmission failures, and reverse is usually the first gear...

Q: The door latch mechanism seems to be broken and will not allow the door to open. How do I open the door to replace it?

Hi! You can access the lock mechanism by removing the inner door panel. This should allow you to open the door. This may be difficult to do! If you are looking for some extra help, a certified technician from YourMechanic...

Q: How difficult is it to replace a valve lifter and/or is it something worth taking to a mechanic?

Hello, thanks for writing in. Having lifters and valves replaced is best left to the professionals. Many times this can involve removing the cylinder head from the engine which will have an affect on the timing. Special procedures need to...

Related articles

What Causes Hoses to Leak?
While the largest part of your engine is mechanical, hydraulics plays a significant role. You’ll find fluids at work in a number of different areas. Your car's fluids include: Engine oil Transmission...
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...
P2428 OBD-II Trouble Code: Exhaust Gas Temperature Too High Bank 1
P2428 code definition A P2428 trouble code signifies that the PCM has detected a problem in the exhaust gas temperature sensor circuit in bank 1, which subsequently contains the number one...