- Bad engine mounts -- those worn or damaged to the point that they can no longer support the engine or control engine torque -- will inevitably result in excess engine movement. Depending upon your engine, the type of crankshaft it has, how and if it's balanced and the engine firing order, minor oscillations in the motor may end up working their way through the softened mounts and into the frame. On the other hand, soft engine mounts may actually reduce minor engine vibrations but still allow the solid thumping of a rocking engine to work through the frame.
- Engineers account for a certain amount of engine movement when designing electrical connections, plumbing fixtures and exhaust systems. But this movement is generally limited to no more than about 1/2 to 3/4 inch in any direction. Bad motor mounts can easily allow an engine to rock several inches back and forth in the engine bay; at the very least you're looking at loose electrical and exhaust fittings, but you may wind up with a whole host of loose or broken wires, hoses and harnesses depending on what has the least amount of slack in the line.
- On many cars, front-wheel drive in particular, the engine and transmission actually form a vital triangulating cross member of the chassis. Without a solid link between the body and the chassis, the nose of the car can twist and move laterally relative to the body. Since the suspension connects to this front clip, bad motor mounts can easily result in squeaks and rattles over bumps, sloppy or vague handling and an increased tendency toward under-steer or "push" while cornering.
- The next step up from minor chassis twisting is, logically, major chassis twisting. Constant twisting back and forth will weaken the welds in your chassis, causing a great deal of unseen structural damage that may be nearly impossible to fix. Once you get to that point, expect constant squeaks and rattles, extremely sloppy handling, leaking weatherstripping and possibly even cracked windows. On some cars, it is hypothetically possible that the engine could literally fall out of the car, but most of the time either the cross-member or the engine mount through-bolts will catch it.