The most common signs and symptoms of failed, defective, or damaged engine mounts are largely similar across all vehicle makes and models, although the severity of some symptoms may vary depending on both the vehicle, and the nature of the mount failure. Nonetheless, some common symptoms of failed or damaged engine mounts could include one or more of the following-
Excessive engine movement
Since engine mounts have the task of preventing excessive engine movements, especially during acceleration, worn, damaged, or broken engine mounts can cause excessive engine movements in more than one direction. Note though that both the degree and direction of excessive movement depend on the engine in the vehicle, as well as the type and location of the failed engine mount.
Loud “thumping” or knocking sounds under some conditions
Depending on both the type of engine mounts in the vehicle, as well as the nature of the failure, damaged engine mounts can cause metal parts of either the mount itself or metal parts of the mount and the engine or transmission to contact each other during excessive engine movements.
It should be noted that in severe cases, or in cases where a defective engine mount is not replaced promptly, the continued knocking of mounts and parts of the engine or transmission against each other could knock holes in the engine or transmission. This usually leads to massive losses of lubricants, which could cause fatal damage to both engines and transmissions to occur in very short order.
Excessive vibration in the vehicle
Even in cases where metal parts of a failed, worn, or damaged engine mount do contact the engine or transmission, the damaged mount will not be able to damp out vibrations originating from either the engine or the vehicle’s structure. In such cases, the reduced effectiveness of a damaged mount will allow most, if not all of the engine’s vibration to pass directly into the passenger cabin via the vehicle’s structure.
Broken or dislodged radiator hoses
Although radiator hoses are designed to accommodate some engine movement, excessive engine movements caused by severely damaged engine mounts can stretch radiator hoses from the engine to the radiator, which is fixed to the vehicle's bodywork, to the point where the hoses can either slip off their attachment points or break under the strain that excessive engine movements place on them.
Either condition leads to an explosive loss of engine coolant, which could cause fatal engine overheating damage to occur if the engine continues to run for more than a few seconds after the loss of engine coolant occurred.