Symptoms and Replacement of a Failing Engine Mount

a new engine mount
What does this new engine mount do?.

Considering the engine is a mass of moving parts, some vibration is to be expected. To minimize noise, vibration, and harshness (NVH) the engine is not mounted directly to the frame. Instead, the engine is held in place by soft rubber or hydraulic engine mounts. "Engine mounts" may be called motor mounts or transmission mounts, depending on their location. The soft material absorbs shock and vibration, making for a more comfortable ride. Here, we're addressing three points:

  • How engine mounts work
  • How to tell if an engine mount is failing
  • How to replace a broken engine mount

How Engine Mounts Work

dog bone engine mount
This “dog bone” engine mount keeps the engine from rocking back and forth too much.

There are three types of engine mounts, and each differs in form and function. They all prevent engine vibrations from transmitting to the rest of the vehicle. Automakers might use one or all these types, depending on the application.

  • Passive rubber mounts are the most basic. These mounts have metal mounting points, which bolt to the engine and to the frame or body. Heavy rubber blocks separate the metal parts. This allows for a small amount of movement without transmitting NVH to the frame or body. Rubber mounts may last the life of the vehicle.
  • Passive hydraulic mounts are slightly more complicated. Like rubber mounts, they are constructed of metal and rubber, but with hollow chambers. The hollow chambers are filled with hydraulic fluid, usually glycol or hydraulic fluid. Hydraulic mounts tend to transmit less vibration than rubber mounts, but may not last as long. They are also slightly more expensive than rubber mounts.
  • Active hydraulic mounts are like passive hydraulic mounts. In addition to fluid-filled chambers, active mounts also have a vacuum chamber. Engine vacuum is applied to the chamber, controlled by a vacuum switching valve (VSV). This enables the mount to adjust its stiffness, depending on the situation. At engine idle, the VSV opens, "loosening" the mount to absorb more vibration. As engine speed increases, the mount "stiffens" to absorb less vibration. Some active hydraulic mounts have their own controllers. These can induce a "counter-vibration" to further reduce engine vibrations. Because active mounts are expensive, be sure to check their vacuum lines and VSVs before condemning the mount, itself.

Symptoms of a Failing Engine Mount

a car overheating
A failed engine mount could lead to broken hoses or wiring.

Knowing what engine mounts do can make it easier to recognize when they fail. Generally, more engine vibration is a good indicator of a failing engine mount. There are three basic symptoms to look, listen, and feel for.

  • Excess Vibration should be one of the first things that you notice, but it might go gradually. This is typical of hydraulic mounts and active mounts, but it can happen to rubber mounts, too. When the mount fails, it'll transmit more vibration to the rest of the car.
  • Excess Movement might be noticed if you look under the hood and rev the engine. A broken engine mount won't let the engine fall out of your car. Still, it could cause damage to other parts, like coolant hoses, exhaust pipes, or a wiring harness.
  • Clunking Sounds are a good indicator that the engine is loose. This usually occurs on hard acceleration or when engine braking. Again, the engine won’t "fall out," but could cause damage to other parts.

How to Replace an Engine Mount

a broken engine mount with the center missing
This broken engine mount won’t drop the engine, but let it move too much.

Fortunately, replacing an engine mount is not complicated. One could simply say "remove and replace," but it's not that simple. The engine mount is held in place by nuts and bolts. There may be two to over six fasteners, as well as vacuum lines – for active mounts.

  1. Support the engine. You may need to lift the engine, using a jack or engine hoist. Kudos if you have an engine holder. Even a 4x4 and a ratchet-strap might be enough.
  2. Loosen the engine mount and lift the engine off it. On active mounts, mark the vacuum port hoses. This will help you get the lines to their respective ports, later. Then remove the old mount and discard.
  3. Install the new engine mount. Be careful not to damage vacuum ports on active mounts. Install all fasteners finger-tight, then allow the full weight of the engine to rest on the mount. Torque all fasteners, then connect vacuum lines to active mounts.

Note: Because active mounts tend to be expensive, you may consider replacing it with a passive mount. This may result in additional vibration but is perfectly safe.

As you can see, engine mounts are not all that complicated. When diagnosing engine vibrations and noises, check the engine mounts. Finally, other factors may be responsible for premature engine mount failure. Abuse, racing, and balance problems are just a few of these. In this case, consider stronger performance mounts, if available.