Why do Engine Mounts Fail?

Although engine mounts on modern vehicles typically have service lives of up to ten years, failures like the one circled in red in the above example can occur for a variety of reasons that include the following-

Long use

The compounds that make up the absorbing cushions in solid engine mounts like the examples shown above are designed to flex billions of times over their useful lives, even though the amounts of flex they are subjected to are typically very small.

However, engine mounts of the type shown here are not loaded equally: in this example, the mount is loaded along the two axes shown in blue and yellow, but it is worth noting that this particular mount is loaded more heavily along the yellow axis than along the blue axis. Thus, since the weight of the engine is concentrated in the area circled in red, the absorbing cushion was more heavily loaded at this point than at any other, which greatly increased the negative effect of millions of extensions and contractions of the rubber compound at this particular point.

In technical terms, this kind of failure is known as a stress fracture, and while these failures occur at different points in different mounts, the cause in all cases is always related to the way the engine mount is loaded in the vehicle.

It is worth noting that once a stress fracture occurs in an engine mount of this type, the loads on the other weight-bearing points in the absorbing cushion are not only increased; the loads are also more unevenly distributed, thus causing other load bearing points in the mount to fail along the axis that is the most heavily loaded. In the example shown above, the blue axis bore most of the loads after the point circled in red failed, which explains the stress-related failures at both points along the blue axis.

As a practical matter, it could be said that stress fractures in solid rubber mounts are a defining characteristic of this type of engine mount, and such fractures do, therefore, not necessarily mean that there is anything wrong with the vehicle.

Hydraulic leaks in active mounts

This type of failure occurs when the hydraulic fluid or gel in an active engine leaks out as a result of damage occurring to the flexible envelope that contains the fluid or gel.

Typical causes of such leaks include long use of the mount, excessive shock loads to the mount as the result of an accident or striking a pothole or other obstacle, as well as incorrect or sloppy installation of an active engine mount.

In some cases though, failures of active engine mounts occur when the solenoids or valves inside the mount that control the mounts’ damping action fail, break, or stop working for reasons that may not always be clear. This means that although there may not be visible leaks of fluid or gel, the failure of an active engine mount’s internal components will cause the mount to stop working, or seriously reduce the effectiveness of an affected active mount.   

Engine and/or transmission oil leaks

Image shows the damaging effect of oil leaks on engine mounts.

This image shows the location of an engine mount as seen from under the vehicle looking up but the important thing to note here is the presence e of engine oil on the mount where it attaches to the engine.

In this example, the rubber-containing absorbing cushion has been destroyed by contact with the oil; in fact, this mount has been destroyed to the point where the mounting bolt no longer passes through the centre of the mount, as shown by the yellow line that denoted the centre line of the mount.

As a practical matter, though, it should be noted that all compounds that contain natural rubber, as most engine mounts do, are subject to chemical attack by hydrocarbons in engine and transmission lubricants. Note also that once oil comes into contact with a rubber compound,  the oil will continue to attack and degrade the rubber, even after all visible traces of oil have been removed.

Thus, the only reliable and long-term remedy for oil-contaminated engine mounts is to repair the oil leak before replacing the damaged engine mount. Failing to repair the oil leak will just destroy the replacement mount as well.

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