My Car Has An Idle Problem

how to clean a throttle bodyI have a Pontiac Grand Prix. Have you heard about or if you know anything about an idling or fading out problem when starting the engine of the Grand Prix. I was reading some reviews and seem several comments were in that area and I seem to have that problem also.

Any information about that would be helpful. I do not get the impression it is a correctable problem.


Hey Nancy,

Most erratic idle, slower and extended starting, and dying at idle or slow speeds complaints are usually caused by a dirty throttle body and idle control motor. You can have your mechanic, or even a fast lube place clean both items for you inexpensively and quickly.

This service should be part of your regular routine maintenance program anyway regardless of it correcting this specific problem or not.

The throttle body is basically where the fresh air under the hood from the air cleaner enters the engine to be mixed with the gasoline. Inside the throttle body is a large valve, or “vane” that opens and closes as you push on the accelerator pedal inside the vehicle.

Throttle body

Picture of a common car throttle body. Click to enlarge

Over time, that vane becomes gunky and sticks inside the throttle body. When that happens the air fuel mixture is incorrect and the computer on the vehicle tries to over compensate for it. When that happens, you will get too much fuel for a second, then too little for a second as the computer tries to make adjustments.

This cause the idle speed to fluctuate and move up and down as you come to a stop or sit at a stop sign. In worst cases, the engine will die because it can not keep a constant idle speed. Rough idle is also felt, the RPM gauge might start to fluctuate as you sit at a stop sign, not enough to cause the engine to die, but enough to FEEL a roughness in the idle speed.

I sometimes will place a Styrofoam coffee cup with a little water in it on the dashboard on in the center console cup holder to witness the rough feeling. If cup or water moves/vibrates I will clean the throttle body and see if that makes things better.

Broken engine and motor mounts will also cause rough idle issues. The “mount” is a block of rubber held together with two pieces of metal, kinda like an Oreo Cookie, the white part is rubber. Over time that rubber breaks, cracks, comes apart etc. and the cushion that it was offering the engine is no longer there.

Then, two pieces of metal are now either touching each other causing a clacking noise or since the rubber inside piece is no longer there…the engine is no longer connected to the other side of the mount. Without that rubber inside piece the engine vibrates, and will physically move up and down as you accelerate.

You can check the mounts by opening the hood, standing to the side of the vehicle, having someone inside the vehicle put the car in reverse WITH ONE FOOT ON BRAKE give the engine a few SMALL pumps of fuel on the accelerator. If the engine moves up and down more than 3 inches the mounts are probably bad and need to be replaced.

Here is a video showing you more about engine mount testing

Please share this with your friends,
Austin Davis

Posted in: Won't Start Issues

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