January 13, 2012

Car Talk

Do motor mounts really need to be replaced?

Syndicated columnists

Dear Tom and Ray:
The Toyota dealer where I have my car serviced recommended that I replace the motor mounts. Since I had never heard of motor mounts breaking, except maybe in an accident, I declined. They claim the engine is jumping around and that it is hard on the belts, etc. When I open the hood with the motor on, my engine seems stable and quiet. Do I need to find a new mechanic? — Marsha

Ray: No. Your mechanic probably is right, Marsha. Camrys are famous for having motor mounts go bad. And if your car is six or seven years old, it wouldn't surprise me at all if you need new ones.

Tom: They don't just fail in collisions. They're under stress all the time, and eventually, motor mounts just wear out.

Ray: When your engine is just idling, and not trying to move the car, it probably does sit quietly. But when it's delivering its "twisting power" to the drive train, that creates one of those Newtonian situations where every force has an equal and opposite reaction. And if the crankshaft is twisting one way, the engine tries to twist the other way.

Tom: The job of the motor mounts is twofold: One is to hold the motor in place. The other is to provide some isolation between the engine and the passenger compartment, to cut down on vibration. So if the mounts are broken, you're likely to feel more vibration from the engine, you might hear the engine bump around when you go over bumps, and when you step on the gas, the engine literally will try to lift up out of the engine compartment.

Ray: That's not necessarily bad for the belts, but it can cause other problems. In an extreme case, the engine can move so far that the axles can fall out and cause the transmission to leak.

Tom: But just looking at the engine while it's idling is not an adequate test for the mounts. The test we do is that we find someone brave or dumb enough (e.g., my brother) to stand off to the side of the engine with the hood up.

Ray: And then a mechanic will plant his foot on the brake, put the transmission in gear and give the engine some gas without letting the car move. That "loads up the engine" and simulates the conditions that take place when the engine is trying to move the car. And if the motor mounts are bad, you'll see the engine jump up several inches.

Tom: At which point my brother tries to catch it. No, not really. If the mounts are good, the engine shouldn't move more than an inch or so.

Ray: So go back and ask the dealership if they'll do that test for you. My guess is that they've already done it. But if you see it for yourself, you'll feel a little less bad about spending hundreds of bucks for new mounts. Good luck, Marsha.

(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk website.)

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