Internal combustion engines produce a large amount of movement and vibration while running. As engine output -- torque and horsepower -- increased throughout the years, automakers had to focus on regulating the engine's movement somehow. The solution was a set of motor mounts constructed of rubber and metal. These motor mounts performed more functions than just holding the engine in place, and a broken mount can result in a rather uncomfortable ride.
Motor mounts play a critical role in keeping your vehicle running smoothly. A motor mount has a metal frame and a rubber insert or inserts. A bolt secures the bracket on the engine to the rubber insert. The rubber insert's main function is to hold the engine in place, but it also allows the engine to move slightly under torque and absorbs engine vibration.
Some high-end vehicles use hydraulic motor mounts. This type of mount performs the same function as a standard motor mount, but it has a hydraulic cylinder instead of a rubber insert.
A motor mount fails in three stages. The first stage is weakening of the rubber insert. This allows the engine to move more than it should. This is caused by either age or deterioration from exposure to oil, and may result in a mild vibration at idle.
In the second stage in the progression the motor mount insert develops a large crack or completely deteriorates. At this point, the vehicle's engine rests on the motors mount's metal frame, resulting in severe vibration and noise.
In the final stage the frame breaks. This occurs from the repeated impact of the engine against the frame. This results in severe vibration and loud noises from the engine compartment.
When you feel a vibration caused by a failed motor mount, you must diagnose which mount has failed. Most engines have two motor mounts -- one on each side of the engine, and a transmission mount. You can find which mount has failed by putting the vehicle in drive or reverse, and having an assistant hold the brake pedal as he lightly presses the accelerator pedal. If there is a failed motor mount, the engine will rock back-and-forth violently. A correctly functioning motor mount allows the engine to move, but the movement is slight and smooth.
The motor mounts help keep the engine, transmission and related components lined up. If an engine mount fails, resulting in the engine sagging, the components no longer line up. This failure may result in severe component failure, including, but not limited to: CV axle failure, transmission failure, exhaust leaks, flex-plate or flywheel damage and internal engine failure.
If the engine sags far enough, due to a failed engine mount, it can cause added tension in the accelerator cable. This tension can open the throttle body, leading to the vehicle accelerating. This unexpected acceleration can lead to an accident.
