Sermon of the Mount

I’m going to categorize motor mounts into two basic groups: hydraulic and non-hydraulic (elasto-rubber). Before I give a basic explanation of how they work, a short explanation of their purpose is in order. Motors and transmissions (the powertrain) can vibrate when spinning. Inside are large masses of reciprocating and rotating weight. If the heavy powertrains were mounted directly to the vehicle’s frame, the vibrations would radiate through the frame and into you. Also, if the powertrains were solidly mounted, lack of rigidity would eventually crack the frame or powertrain member. Therefore, powertrains are mounted with rubber. The rubber is elastic and stretches and compresses to absorb powertrain vibrations. A traditional motor mount has an upper and lower steel member with a block of solid rubber sandwiched in-between. A hydraulic motor mount is basically the same except the solid rubber block is a rubber bladder filled with hydraulic fluid. The hydraulic mount is generally larger and more expensive. The upper steel member is bolted to a powertrain member and the lower is bolted to the frame.

Hard rubber motor mounts are cheaper but don’t absorb as well. Hydraulic mounts do a much better job but they are less durable. Some problems with the solid rubber types of mounts are, depending on which type, the rubber sandwiched between the two steel members is eventually compressed by the Sermon of the Mount weight of the engine or the heat from the exhaust causes the rubber to harden and lose elasticity. Some rubber motor mounts are designed like suspension bridges. The motor is bolted at the midpoint of the rubber bridge. Over time and loads, the bridge sags and rests on the frame. It’s almost like having no motor mounts at all. The main problem with hydraulic mounts is they tend to crack and leak. As I mention in the beginning of this article, some hydraulic motor mounts are controlled by the engine computer. Honda, for example, has a V6 variable displacement engine that when it’s in “Eco Mode” drops out three cylinders. The motor becomes a three cylinder as the vehicle cruises down the highway. The three remaining cylinders are all in the same bank. This odd fire configuration creates a motor vibration. The PCM creates a counter vibration in the hydraulic motor mount to counter balance the odd fire vibration.

Next, we go to an open space with nothing around to hit. We raise the hood, set the parking brake, place wheel chocks at every wheel and press as hard as we can on the brake pedal. If we have an automatic transmission, we “Power Jack” (step on the brake pedal and gas pedal at the same time) the motor. If you are doing what we do, be careful and start easy. Watch and see if the motor raises. CAUTION, if a motor mount is severely damaged the motor can raise so high it may look like it’s going to leap into your lap. BE EASY, BE CAREFUL. Do the same in reverse and drive so you can load the motor mounts on each side of the engine. The motor should not rise or twist. Also. if during the test you hear a thud, it’s time to change motor mounts. If you drive a manual transmission, it’s much harder to Power Jack. You will have to slip the clutch. This is tough to do. Just try it a couple of times and then rest before trying again. I don’t want you to smoke the clutch assembly.

Changing motor mounts can be a very frustrating job. Lots of tight spaces, reaching around narrow corners, sharp edges and dirty. It’s a real knuckle buster. Don’t snap off any bolts or you will have a very, very bad week. That’s what happened to us. Having a left hander available is very helpful. Lefties have no problem reaching around places backwards to us. I don’t personally replace motor mounts any more, I pass that job off to my staff. I still have the hole in the wall inside my shop from 25 years ago when my wrench flew through the sheet rock and into the wall. Wrench is still there.

Guys, LISTEN UP: if anyone driving the vehicle you intend to replace motor mounts on is annoyed by little things, use the factory original equipment mounts. You don’t want to do this job a second time. If it’s your work truck and you want to save money, after market mounts should be fine. NAPA has a new line of quality motor mounts that have been successful for us. I still will not use anything but original equipment mounts if they are tough to replace. OK, let’s get to replacing. Raise the vehicle as high as you can and put it on four jack stands. Make sure you compare the new mounts to the old ones and make sure they are a match. If not, you can waste a lot of time trying to install a nonmatched look alike part. Don’t force the motor mounts in. They should fit. If you force them, they are in a bind and you will create a new vibration.