Typical cause is a broken belt in a tire. If it only does it up to about 35 mph, and stops shaking when you let off the gas, suspect a worn inner cv joint, but this isn't common on an Avenger; more common on minivans.
First look at the two front tires and look for a hump in the tread. The hump can often be felt by running your hands over the tread, but BE CAREFUL; often there will be wires from the steel belts sticking out and they really smart when you poke yourself. I'm a chicken now, so I pat my hands over the tread rather than rubbing the tread.
Another method is to jack the front end off the ground and run the car in 1st gear, then watch the tire tread from in front. The tread should stay nice and steady as the tire rotates. If you see a noticeable bump, you found the broken belt. Usually just one tire will spin. Block that one to make the other tire spin. Be careful if you block the tire while it's spinning so it doesn't shoot your block out. It's safer to have someone hit the brakes, then block the tire.
The last thing that is much harder to spot also involves watching the tread while the tire rotates, but if the tread itself looks ok, look closely at the bottoms of the grooves in the tread. A noticeable high spot there also indicates a broken belt. It's not as severe, and as the hump grows over time, the tread wears flat as you drive. This is a hard one to find but usually the hump is pretty easy to see.
caradiodoc
Apr 15, 2009 at 2:07 AM
