Whether they are fluid-filled, polyurethane or traditional rubber, a mount's job is to support the weight of the engine or transmission and to prevent unwanted vibrations from snaking their way into the cabin. When these important connectors fail, they can make for a noisy, uncomfortable and possibly dangerous car.
Shaking on Start-Up
If you feel a steady and monotonous lurching when engaging the starter on your car, you're experiencing one of the classic signs of a bad motor mount. On front-wheel-drive cars, this rocking will manifest itself as a back and forth motion. Rear-drive cars will rock from side to side. The rocking settles into a kind of steady vibration when the engine does start, and you might experience a quick lurch when you shut the ignition off.
One of the most obvious symptoms of a badly damaged engine or transmission mount on a front-wheel-drive car is a hard thump on acceleration or hard braking. This is usually more symptomatic of a front or rear transmission mount than a motor mount, and sounds like a snowball or heavy clump of dirt hitting the undercarriage. This type of failure usually isn't dangerous, but can result in broken parts or sheared bolts later.
Because the engine and transmission in a modern unibody car essentially functions as a structural member, cracked or worn motor mounts will inevitably result in severe chassis flex. A flexing chassis can result in poor handling, creaking weather-seals and door hinges, and cracked strut-tower braces. Over time, an overly flexing chassis can permanently bend sheet metal and cause misalignment of bonnet and door panels.
Though engine and transmission mounts are designed to allow a certain amount of movement, an engine that visibly rocks more than about a half an inch is a sure sign mount failure. If the engine moves enough to cause a tennis ball to roll off of the air cleaner when the vehicle is placed in drive, then more than likely you have a bad mount. More often than not, such a degree of engine rock usually indicates a bad transmission mount.
Because excess engine movement places stress on every component connected to it, observing the condition of the wiring, hoses and linkages can be a good indicator of mount condition. If you see any wires or hoses go taut when the vehicle is placed into drive, then more than likely, one or more mounts is in need of replacement.