I have a 2004 Grand Prix GT1 with about 97,000 miles on it. At highway speeds my front end shakes/vibrates. I can feel it in the steering wheel, at my feet, and both the driver and passenger seats. When I let go of the steering wheel, it shakes horizontally (clockwise and counter clockwise) about a quarter inch or less. It doesn't shake insanely, but it is definately visually noticable. The shaking/vibration seems to get worse when the speed increases and when I accelerate, but more so when I lightly accelerate. If I'm going 70 mph and let off the gas, the shaking/vibration lessens. If I'm going 70 mph and lightly accelerate, the shaking/vibration worsens/increases. I did a road test with a friend today and had him takes notes while we were driving. We both felt it from 40 mph and up. It isn't as noticable under hard acceleration as it is under light acceleration.
It isn't a wheel balance issue because I just had all 4 wheels rotated and balanced. About 6,000 miles ago I had all 4 wheels rotated and balanced because I was noticing it back then. I do have one slightly bent wheel, but the bent wheel has had a chance to be both in the front and the back. I have also had the tie rods, front wheel bearings, and intermediate shaft replaced since I have owned the car (I bought it 53,000 miles ago).
Is it a tire issue? I did the quarter test and on the outer sides of the tires the tread comes up to Washington's head. In the middle of the tire the tread comes up past his head. I assume that's fine. I did get the tires 50,000 miles ago. Time for new ones or is the tread fine? My mechanic hasn't mentioned any signs of uneven wear except for the tire on the slightly bent wheel. He says it looks like someone hopped a curb at one time. Probably my girlfriend, haha.
I did see something on the internet about GM 3.8L vehicles having problems with the CV shaft and it causing shaking/vibrating at highway speeds and under acceleration. Is this common for our Grand Prixs too? Could this be the culprit?
I did pop the hood and looked at the engine. It doesn't look like it is a motor mount issue. The motor barely moves when the car is in nuetral and gas is given to it. When shifted into drive or reverse, it looks normal. Also, I don't think it's an allignment issue because the car tracks straight while going down the road.
If you guys can give me some help, information, tips it would be greatly appreciated! I have a 100,000 mile bumper to bumper warranty and I would like to get this issue fixed before the warranty runs out. Thanks guys!