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Mounts Keep Things Quiet Under The Hood

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Engine and transmission mounts are an often overlooked cause of engine noise and vibration. A loose or broken motor mount that allows excessive engine movement may allow a belt-driven fan to hit the fan shroud, or the fan to hit the radiator hoses. Excessive engine movement also can increase the risk of radiator or heater hose failure, and may even cause the throttle linkage to bind or stick.

A bad transmission mount can create driveline noise when starting out or accelerating, or it may even affect the operation of the clutch or gear shift linkage.

Mounts deteriorate with age and mileage, so it’s not unusual to find one or more broken or collapsed mounts in older high-mileage vehicles. If one mount has failed, chances are the others are near the end of their service life and should also be replaced as a complete set.

The engine and transmission mounts hold and support the powertrain while isolating the vibrations produced by the powertrain from the chassis. On front-wheel drive (FWD) cars and minivans with transverse mounted engines, there are left and right engine/transmission mounts (which are usually different), and a third mount above or below to triangulate support.

Upper engine mounts are often called “struts” because they prevent the engine from rocking back and forth as the vehicle accelerates and decelerates. Upper mounts usually have a bushing in each end with one bushing attached to the engine and the other to the radiator crossmember support.

On rear-wheel drive (RWD) cars and trucks, there are usually a pair of motor mounts on each side of the engine (left and right, which may be the same or different) to support the engine, and a single mount under the back of the transmission. The rear mount may be part of a bolt-in crossmember support.

Engine/transmission mounts are usually made of a solid rubber cushion element laminated to a pair of steel backing plates. The backing plates bolt to the engine/transmission and chassis.

Some applications have fluid-filled “hydromounts” for extra vibration dampening. The extra cushion provided by a glycol fluid chamber allows hydromounts to absorb and dampen engine vibrations that would otherwise pass right through a conventional solid rubber mount. Hydromounts are more expensive than solid rubber mounts, and they can leak fluid and collapse after years of service. Conventional solid rubber mounts are available for many of these applications as a lower-cost repair alternative, but the dampening will not be as good as the original-style mounts.

Some late-model import vehicles have electronically-controlled “active” mounts that can alter the stiffness of the mount to cancel out harmonics at various engine speeds and loads. Some of these use a vacuum-actuator to change the stiffness of the mount. Some mounts even generate their own “counter shake” to offset engine vibrations at various speeds.

Customers who are replacing an engine, transmission or clutch should always inspect all of the engine/transmission mounts, and replace any that are broken, loose, cracked, leaking, collapsed or severely corroded. New fasteners also are recommended.

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