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Q: I own a 2005 Buick LeSabre with 65,000 miles on it. It has developed a vibration when stopped at a light with my foot on the brake and the transmission in drive. The vibration can be felt in the floor, brake pedal and steering wheel. Three GM dealers and one independent service agency have worked on the car. All four engine/transmission mounts have been replaced but the car still vibrates. Can you offer any suggestions?

A: While the engine/transmission mounts are always suspect in a case like this, there are several other possibilities. An engine misfire, roughness or vacuum leak can cause an idle shake. Since the power brake system utilizes engine vacuum for its assist, perhaps vacuum is leaking only when the brakes are applied.

In a safe location such as a large, empty parking lot, bring the car to a stop and keep your foot on the brake. Then apply the parking brake very firmly. Release the brake pedal — hopefully the car won't move. Does the vibration/shake stop? If so, a vacuum leak is highly suspect.

Another possibility is contact between a driveline component such as the exhaust system and the chassis. A physical inspection might reveal rub marks confirming the contact.

Q: I have a 2001 Pontiac Grand Prix with front-wheel drive. The two front tires are P205/65-R15 in excellent condition. Several days ago I installed two P205/70-R15 tires in the rear, also in excellent condition. Will the slightly different tire size interfere with handling or fuel mileage? To my untrained eye, maybe this wasn't the best choice.

A: I agree — putting different-sized tires on the rear of your vehicle wasn't the best idea, but it probably isn't a complete disaster. First off, since the larger tires are on the rear of your front-drive vehicle, there shouldn't be any effect on fuel mileage.

The differences in tread width and rolling diameter between the two are relatively small. The rear tires are eight-10ths of an inch larger in diameter when new, but have the same section/tread width, 8 inches. And since the speedometer read-out is based on driveline and front-wheel speed, there won't be any change in speedometer accuracy.

Handling? That's another issue. But while it's never a good idea to mix tire sizes, since the tire size differences are very small and all four tires are in "excellent" condition, I can't see a huge issue.

Q: I drive a 2001 Hyundai Elantra with 178,000 miles on it. Over the past couple of years all four wheel bearings have been replaced. The first two only lasted 1 to 2 days each. The third, replaced along with the drive axle, lasted until April and the fourth went out in mid-May. The replacement lasted until July. What's going on?

A: Most manufacturers of front-drive vehicles utilize sealed front hub/bearing assemblies that are not serviceable. Hyundai, on the other hand, used a non-sealed hub with replaceable bearings in that vehicle.

Since the bearings were replaced individually, the quality and source of the bearings and the quality of installation are suspect. If the hub/bearing/knuckle assembly is not set up and installed correctly, bearing life will be dramatically reduced.

In addition, in July 2001 Hyundai began installing front-wheel bearing dust covers to keep road debris and contamination from getting into the bearings. If your vehicle was not fitted with these, the dust covers can be added when the bearings are replaced.