A Budget-Buster Just Waiting to Happen: When Your Motor Mounts Go Bad
Motor mounts are about as unglamorous a piece of hardware as you can find on a vehicle, seldom listed as a maintenance item in owner’s manuals and never mentioned in new-car showrooms as a reason to buy a particular model.
But they can pack a powerful punch to the wallet when they deteriorate and eventually crack, sight unseen, deep inside the engine compartment. Then the car’s owner will discover the terrible truth: Not all motor mounts are created equally by the masterminds of the auto industry.
Motor mounts connect the engine to the frame, acting as little rubber doughnuts that absorb the bone-rattling vibrations the engine produces and allow a little give to the foot-pounds of torque created by the crankshaft.
“They have to allow the engine to flex in the engine compartment; otherwise, that vibration transfers to the rest of the car,” said Steve Mazor, principal automotive engineer at the Automobile Club of Southern California.
When motor mounts fail, the repair bill can easily surpass $500. In some cases, the power-steering pump, the air-conditioning compressor, heating lines, radiator hoses and other greasy items under the hood must be removed before the real work can begin. The engine must be lifted off the car’s frame and new mounts installed, requiring time-consuming--and costly--labor.
Typically, one motor mount fails before the others. As an engine revs, it wants to rotate in the same direction as the crankshaft. That tends to stretch one motor mount and compress the others. Usually, the mount that stretches is the one that fails first, Mazor said.
Can a failed mount let an engine fall right out of the car? That’s almost impossible, because most engine compartments are shaped like a V and will hold in the engine. Additionally, modern mounts have safety catches that prevent complete failure.
But as a mount begins to fail, it enables the engine to rotate and move inside the engine compartment. And that can cause serious problems.
In some cases, the movement of the engine can pinch hoses and wires. As the mounts continue to deteriorate and allow even more movement, that can cause stresses to build in other areas and sometimes leads to cracks in the transmission housing. On front-wheel-drive cars, the extra movement may cause the axle to fall outside the transmission casing.
In older cars with mechanical throttle linkages, the movement could jam the throttle wide open and put the car in an uncontrolled acceleration. Modern cable-operated linkages have pretty much eliminated that risk.
Mounts have improved somewhat through the years. During the 1980s, the industry introduced mounts that help limit engine movement once the rubber fails. Some mounts have hydraulic cores in which a heavy fluid helps damp engine vibrations. But they too can fail.
Mounts fail for many reasons, including bad engineering and conditions inside the hood. The environment around the engine is terrible for rubber: hot, smoky, greasy and oily, master mechanic Sam Memmolo said. If leaking oil saturates a motor mount, it can quickly destroy it.
The vehicles most afflicted by motor-mount problems are the ones with the heaviest engines, though common sense would suggest that engineers would equip heavier engines with stronger mounts. But the 3.8-liter engine Ford Motor Co. uses in a wide variety of its cars and trucks and some General Motors Corp. vehicles with V-8s are known for motor-mount nightmares, Memmolo said.
Finally, every motorist should know something about diagnosing a motor-mount failure, just in case somebody wants to replace yours. Mounts generally are not visible, though in some cases they can be observed from under the car. Cracks and bulges are a bad sign.
If you can feel the engine shift or clunk when you accelerate or put the car in gear, you may have a problem. You can observe the engine with the hood up and have somebody else shift it from park to drive. Expect some movement, but if you hear any unusual clunking, you have a problem.
As with every other repair, you need a mechanic you can trust.
Ralph Vartabedian cannot answer mail personally but responds in this column to automotive questions of general interest. Please do not telephone. Write to Your Wheels, Business Section, Los Angeles Times, 202 W. 1st St., Los Angeles, CA 90012. E-mail: firstname.lastname@example.org.