Motor mounts support the engine and transmission, and dampen noise and vibration. The mounts isolate engine and transmission from the chassis so vibrations and noise are not transmitted to the rest of the vehicle. On some front-wheel drive cars, upper mounts (torque struts) also control the fore and aft movement of the engine during acceleration.
Most motor mounts are relatively simple in design and consist only of metal attachment plates and large rubber insulator blocks. The rubber portions of the mount are flexible and provide the cushioning that dampens engine vibrations. The metal bracket part of the mount provides the mechanical support and attachment points for the motor mounts.
Some vehicles have hydraulic, hydroelastic or hydro-mounts with hollow chambers filled with glycol or hydraulic fluid. Hydro-mounts act like a jelly-filled donut to absorb vibrations that would otherwise be transmitted to the chassis.
Hydro-mounts are often used with four-cylinder and V6 engines that don't idle as smoothly as a V8, and in luxury vehicles where motorists expect less noise and vibration. Some hydro-mounts even have internal valving and/or a solenoid to change the dampening characteristics at different rpms to better tune out unwanted vibrations. These are called switchable hydraulic mounts or electronic mounts.
To the left is a cutaway of a hydraulic motor mount showing the fluid-filled hollow chambers inside.
Some late model vehicles have "active" motor mounts that can change stiffness in response to engine speed. On Lexus, Toyota and Honda applications, the engine computer energizes a solenoid to apply engine vacuum to an internal chamber inside the motor mounts. When vacuum is applied, the mount becomes softer to provide additional dampening of engine vibrations (typically at idle).
Delphi has recently developed magneto rheological motor mounts called "MagneRide" engine mounts that use a magnetic fluid to vary the dampening characteristics. The mounts contain a fluid that has iron particles suspended within the fluid. When an electrical current is applied to the fluid, the particles line up and increase the viscosity (stiffness) of the fluid. This makes the mount firmer.
When a motor or transmission mount fails, one of several things can happen. If the rubber separates or delaminates from the steel, the mount can break. The design of the mount usually prevents the engine from falling out of the car, but it cannot keep the engine from twisting or rocking when your vehicle accelerates or is under load. This can produce thumping and rattling noises as well as overstress components such as radiator and heater hoses, wiring connectors and the exhaust system. In rear-wheel drive applications that have an engine-driven fan, a broken mount may allow the fan to hit the radiator or shroud. Drive belts or pulleys may also be forced to rub against other components if clearances are tight.
A broken or loose motor mount in a front-wheel drive application can be even more serious because it may allow engine movements that interfere with the throttle or shift linkage. Excessive fore and aft rocking of a transverse-mounted engine can also lead to exhaust leaks where the head pipe joins the manifold or cause the head pipe itself to fail. If the broken mount is an end mount, it may also contribute to a torque steer condition and cause accelerated wear or separation of the inner CV joints on one or both halfshafts.
On vehicles that have fluid filled passive hydroelastic motor mounts or vacuum-actuated active motor mounts, loss of fluid leaks or loss of intake vacuum to the mount may increase idle vibration and harshness.
The motor mounts are seldom checked unless there is an obvious problem, and they may even be overlooked if the engine or transmission is being replaced. So always check your motor mounts if the engine seems noisier than usual or you can feel engine vibrations inside your vehicle. The motor mounts should also be inspected when any major engine or transmission work is done, or when replacing a clutch, halfshafts or a driveshaft.
Motor mounts can be visually inspected for cracked, loose or broken brackets, loose or missing bolts, collapsed rubber or fluid leaks (hydro-mounts). A pry bar can be used to check for separated or broken mounts.
Another way to check mounts is to put the transmission into drive and lightly load the engine while keeping your other foot on the brakes. Excessive engine movement may indicate loose or broken mounts that need to be replaced.
Replacement mounts may or many not have the same construction as the original. Fluid-filled hydro-mounts are expensive, so a more affordable alternative may be a solid rubber mount. But a solid mount obviously cannot provide the same level of vibration dampening as an original equipment hydro-mount. Consequently, you might not be happy with the way your car feels if you opt for the cheaper solid rubber mount.
Replacing a motor mount always requires supporting the engine and/or transmission. The weight must be taken off the mount before it can be unbolted and replaced. This can be done with a floor jack from underneath (place a block of wood between the jack and oil pan to distribute the pressure so you don't damage the oil pan), or with an engine hoist overhead. Mount access can also be a challenge on some transverse-mounted engines in front-wheel drive cars.
Other items you may also need when replacing motor mounts include new fasteners, new radiator hoses or exhaust pipe gaskets.