2004 Pontiac Grand Prix- while accelerating at 55+ car shakes? Is this the drive train? Have had wheels balanced already
Could possibly a number of things, mine was the Throttle Positioning Sensor, which controls your fuel mixture, have checked by a mechanic to be sure, also could be clogged fuel injectors.
Does it shake front to back? Or does it vibrate left to right?
The whole car feels like its shaking?
could be out of alignment, or have worn tie rod ends.
I would test your transmission fluid also,along with what Dorian recommended.The car has to be warmed up to test the fluid,run it for about two to three minutes,then turn the car off,check the fluid and if it is a nice pink it is fine,but if it is kind of orange and burnt looking than that might be your problem.Transmission filters aren't that expensive.
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