Car Vibrates when I let off gas?

Question

I bought a 1997 Pontiac Grand Prix (3800 Engine) a few months ago and starting noticing this problem. It only vibrates/shakes when I'm driving up at 55+ MPH. Its fine when I press on the gas pedal but when I start to let up is when it starts shaking. Any ideas on what it could be?

Answer 1

Id have the CV shafts checked

Answer 2

I doubt that it's the CV joints, unless you also hear a rumbling/clunking/clicking noise from them (especially when turning at slower speeds).  Even if they were worn I don't think they'd cause the kind of vibration you're describing.  Look carefully at the rubber boots that cover them (near to where the driveshafts connect to the hubs/knuckles that the wheels are bolted to).  If they're torn, there's a good chance that dirt has gotten into the bearings and they are wearing.  If the boots are torn and you're going to keep the car for a while, it's probably best to get new (or rebuilt - they're cheaper and just as good) half-shafts (the technical term for the two "driveshafts" that come out of each side of the transaxle (transmission)).  Often the rebuilt half-shafts are cheaper than just replacing the CV joints that are attached to them, and of course you would get new rubber boots with them.  Make sure they're greased really well with the correct type of CV joint grease.

I'd say though that the vibration problem is more likely being caused by one or more of the following, in order of probability:

Tires out of balance:  Look to see if there are any balancing weights on them, both inside and outside of each front tire.  If there are none, they were either never balanced, or they have fallen off.

Worn components in the steering linkage:  Tie rod ends, idler arm, pitman arm.  With the car on the ground, see if you can observe play in any parts of the steering linkage when the steering wheel is turned back and forth.  

Worn or loose wheel bearing(s): With the front end off the ground, agressively move each tire back and forth to see if there is excess play in the wheel bearings.  Be careful not to mistake steering linkage play for wheel bearing play.

Worn ball joints:  Again, with the front end just off the ground, put a 2x4 under each tire and try to lever the tire upward; watch for movement at the ball joint on the bottom of the steering knuckle assembly that the front wheels are attached to and pivot on when they turn.

Vibrations are usually worse at certain speeds when the wheel and tire are at just the right speed to set up a "harmonic" vibration; they often go away when you brake or decelerate.

Good luck.

Edit:  The previous answer mentioned the "torch converter".  The correct term is "torque converter", a device that smoothly transmits power from the engine to the transaxle.  They're used only on automatic transmissions.  But his answer raises a good point;  I was thinking along the lines of a front end vibration, but there is a chance that it could be either a loose torque converter or worn motor/transaxle mounts.  The engine and transaxle are attached to the frame or body of the car at three places, where they rest on fairly hard rubber mounts that hold them in place and keep vibrations from being transmitted to the rest of the car.  If they are worn out or loose, the engine and transaxle can shake excessively.  They are checked by jacking up the engine and transaxle at each location while you watch for excess play.

The torque converter bolts can usually be checked (and tightened if necessary) through an access panel, without having to separate the transaxle from the engine.  If you can, ask the previous owner if the engine has ever been replaced.  If not, those bolts are probably fine.

Answer 3

  I agree to a point it could be a CV joint but the 97 grand Ams had a lot of trouble with the 3800s. I would have someone check the bolts between the torch converter and the flywheel. The engine may have been changed and they might have missed  them. Good Luck.