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"Shaking" Maxx

Filled car with gas, drove 9 miles home. Next day drove 14 miles and car shook violently for 2-3 minutes, then smoothed out and ran great. Told I had gotten “bad gas” (water). Added “Heet”. Two weeks later after starting up car from sitting and driving 10-15 miles, car shakes again and then smooths out. Took to dealer, no indication of anything wrong. Same thing continues at about 2 week intervals: car sits, start up, drive 10-15 miles, car shakes then smooths. Finally, car shakes so violently that “check engine” light comes on – back to dealer. Gas cap bad as is car computer. Both fixed. Two weeks later, car shakes again. No one able to diagnos problem; actually, no one particularly interested. Finally trade car in for new Vibe - told new car dealership about problem, but they didn’t seem concerned either. ANY IDEAS???

Was the car dead cold when you started it? This can happen when the engine is in cold start mode. when the engine warms up it smooths out. Odd interval but it could be due to a flakey temp sensor.

Yes, car would always have been sitting at least overnight. I forgot to mention that when the shaking occurred I could always anticipate it somewhat as the brakes would “tighten up”. When the shaking really got bad, then there was absolutely no play in the brakes - like stepping on a rock - but, the brakes still worked. Scary however. As soon as the shaking stopped the brakes would again have “play” in them.

Well, you traded the car, so even if we think we know the reason you will never know if we were right. How many miles on the car? Were the spark plugs ever changed? What were the codes for the check engine light? I doubt the gas cap diagnosis was right. Did the light come on after the reset?

The connection with the hard feeling in the brake pedal, if it was always connected, suggests a sudden loss of engine vacuum. If it always shuddered in conjunction with stepping on the brake then it suggests a failing brake booster. But if stepping on the brakes wasn’t the “cause” it suggests a sudden loss of engine compression.

What is the year/make/model of the car, and how many miles?

Malibu Maxx - 2005 - 48,000 miles. I loved that car and even though I’ve traded it in, it makes me mad that no one was able to come up with a solution – or even a suggestion of why it shook so. Hope the next owner has better mechanics. Thanks for all your suggestions – they sound better than the ones I received.