Skip the auto shop - Our mechanics make house calls

Q: Rough idle when engine is warm

asked by on

The car runs fine at start up but after 20 0r 30 miles, rough idling start occurring at stop lights. What is the probable cause?

My car has 155000 miles.
My car has an automatic transmission.

This is a common sign of a faulty ignition coil. Ignition coils are coated with a varnish-like insulation that becomes brittle over time after being stretched then contracted repeatedly The insulation develops small fractures that open when heated and close when cooled. When open, they allow shorting of coil windings and decrease or eliminate the coil’s ability to function. This type of temperature cycling happens because every time you shut the engine off, the fan stops running, the coolant stops flowing and the temperature under the hood rises. Every time you start the engine again the coolant flows, the fans turn on, and the temperature drops. When this heat dissipates and is absorbed under the hood, it is trapped causing this cycling effect. Components that contain heat sensitive materials include parts like coils, injectors, igniters, and some sensors. Any of these may be potentially subject to this same kind of heat problem which can be very difficult to trace. I would recommend having an expert from YourMechanic come to your location to take a look at your vehicle to properly diagnose your ignition system.

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: Automatic transmition hit

Having a Jetta in the family, I think I know what you are referring to. When the transmission shifts from 1st to 2nd gear, it shifts very hard. It feels almost like someone bumped into the back of the car....

Q: My car is making a grinding noise when I try to put it in gear and it won't move. noise goes away when clutch isn't pressed down.

Hi Renozwah. Thanks for writing into YourMechanic.com this morning. Generally speaking, there are a few different reasons why your car is making this type of noise while accelerating. First, and typically not considered by many mechanics is that there is...

Q: Oil leak

Hi There, It is quite possible for the oil to take some time to burn off of the engine if it were spilled on it. You may want to (very carefully) spray some degreaser around the engine bay where the...

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...
How Long Does a Heater Control Valve Last?
Keeping the right amount of coolant in a car is essential in keeping the engine at the right temperature. Failing to have the right amount of coolant or even bad elements...
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...