Motor Mount Diagnostics
There was a time when all a motor mount did was hold the engine from twisting out of the engine bay. That’s all changed now. Motor
mounts are still designed to keep all that horsepower from leaping out
from under the hood, but they also work with other functions such as
dampening noise, reducing vibration, ride control, stability control,
and to maintain suspension/component alignment.
Motor mounts are not as simple as they once were. Motor mounts back in the day were just a solid piece of rubber molded to two piece
of metal. Some manufacturers address the problem of the torque side
motor mount failure by adding an overlapping metal section to the upper
and lower metal parts as a way to “catch” the two metal brackets together
if the rubber section came free. This helped prevent the dreaded fan blade in the shroud problem that was always associated with a torn motor mount. Later, as time went on, upper mounts (Torque struts or what are commonly referred to as the “dog bones”) were added to the transverse engines as an extra precaution to engine movement.
The Next Generation of Motor Mounts
A new generation of motors mounts starting showing up in the late 80’s early 90’s. These were a liquid filled motor mount that worked like a miniature shock absorber allowing the engine vibration to be dissipated or at least lowered at the motor mount area. It was a very effective way of lower some of the noise and vibration associated with sitting at a traffic light and watching the gear shift lever rattle. This was a common occurrence on the 4 cyl. and V6 engines that didn’t have the same smooth operation as their big sister the V8 engine. But, the engineers weren’t done yet. There’s more to this motor mount story. Some of the gel filled or oil filled type hydro-mounts were re-engineered with internal valves or a solenoid to change the dampening characteristics at different rpms. Now it was possible to tune the motor mount to the various typical situations that were commonly producing those unwanted vibrations.
Active Motor Mounts
Active motor mounts were first introduced using engine vacuum to change the stiffness of the mount. In most applications the ECM/PCM sent a control signal to a valve (VSV - vacuum switching valve) which then allowed vacuum to reach the motor mount. As the vacuum increased the engine mount was allowed to soften. Typically, the less vacuum that was applied to the mount, the stiffer it will become. The most common problems on these mounts were vacuum leaks caused by a separated motor mount. In some cases, the vehicle may even set a lean code due to the vacuum leak.
Later on, an electrical motor was used to control a valve in the motor mount. Some used a PWM (pulse-width-modulated) signal that could adjust the mount on the spot and cancel out the vibrations. The frequency of this pulse signal is matched to the engine speed to decrease the engine vibration. Since these mounts are generally associated with the PCM data it’s very likely to have a code related to a component or engine condition that is indirectly or directly caused by a failing electronic motor mount. On some models a torque sensor is incorporated into the system to check the actual load or need for mount adjustment and tells the PCM how much adjustment is needed. Codes such as low DC to DC converter enabled circuit, high engine mount control circuit/open, engine mount control circuit, low engine mount control circuit, or high motor torque sensor failure are typically associated to the motor mount system.
Porsche started using an active motor mount with 'magnetorheological' fluid. (Say that three times fast),the same stuff found in some variable shock applications. The system uses magnetized iron particles in a synthetic oil solution accompanied with a small electromagnet. When a current is applied to the electromagnet the viscosity of the fluid inside the mount is rapidly changed to control the movement of the motor mount. At idle the mount can be softer and absorb more of the unwanted vibrations but as soon as you step on the gas the mount can become extremely stiff holding the engine rigidly in place.
Other manufacturers have gone with a ACM (Active Control Mount) system that counter acts vibration by introducing a feedback vibration opposite in amplitude as the vibration the engine is producing. It’s sort of like noise-cancellation headphones, but instead of sound waves that are out of phase to reduce noise, the ACM system reduces the amplitude of engine vibrations via its active motor mount that generates its own counter vibrations.
Motor Mount Checks
Generally, motor mounts are seldom checked unless there is an obvious problem, and are often overlooked if the engine or transmission is being replaced. It's a good idea to remind your customers to be aware and to tell you if their engines seems a bit noisier than usual or they feel an engine vibration that wasn’t there before.
Mounts can be visually inspected for cracked, loose or broken brackets, loose or missing bolts, collapsed rubber sections or fluid leaking from the mounts. A pry bar can be used to check for separated, weak or broken mounts.
Another way to check the mounts is to put the transmission into drive and apply a light load to the engine while keeping the other foot on the brake (power stall). Excessive engine movement may indicate loose or broken mounts that need to be replaced.
Replacement mounts may or may not have the same construction as the original. Fluid-filled hydro-mounts can be expensive, and a lot of times a more affordable alternative can be purchased. But, a solid mount obviously can’t provide the same level of dampening as the original hydro-mount. Consequently, the vehicle owner may not be happy with the way his car feels if a less-expensive solid mount is substituted for a fluid-filled mount. Also, be aware, in some instances, a service light may not be avoided if the mount is directly controlled by the PCM.
Many motor mounts may have an area where you can actually check the condition of the mount without using a pry bar or power stalling the engine. Honda 05-06 Odyssey minivan for example, a defective rear engine mount can be checked with a .024-in. feeler gauge by measuring the clearance between the rear engine mount bracket and the rubber bumper on the mount. If the feeler gauge slides between the two parts or large bulges of rubber are visibly seen it’s time for a new rear mount.
Motor Mount Diagnostics
Typically, when a motor or transmission mount fails, one of several things can happen. If the rubber separates or de-laminates from the steel, the mount can break. The design of the mount usually prevents the engine from falling out of the car, but it can’t keep the engine from twisting or rocking when the vehicle accelerates or is under load.
At idle, this can produce a thumping and rattling noises. Then, as the vehicle begins to move components such as the radiator and heater hoses, air condition lines, wiring harness/connectors and the exhaust system can be pulled or stressed to their breaking point. In rear-wheel-drive applications that use a viscous clutch fan, a broken mount may allow the fan to come in contact with the shroud. Drive belts or pulleys also can be forced to rub against other components if clearances are tight.
A broken or loose motor mount can create even more serious issues if the vehicle is old enough to use a throttle cable vs. an electronic throttle. A torque steering affect from worn motor mounts can also cause accelerated wear or separation of the inner CV joints on one or both half shafts. On front wheel drive vehicles, worn motor mounts can also cause a “crab walk” type of problem. This is when the engine and transmission are mounted on a cradle and the mounts have become soft or torn allowing the engine torque to pull one of the drive wheels ahead of the other drive wheel. This 'crab walking' is sometimes misdiagnosed as a transmission issue, frame alignment problem, or bad tires rather than a motor mount issue.
Summary
With all these changes to the basic motor mount is definitely not just a hunk of rubber that maintains engine stability anymore. It’s incorporated into the electronics of the modern day car just like everything else has. So, it’s not a stretch to say that an engine misfire could be caused by a faulty motor mount. Especially when the motor mount stiffness is being controlled by sensors and is directed to stiffen or become softer by the PCM. It would be no stretch to say that a misfire may not be a misfire at all if the sensor or motor mount is sending back false information to the PCM and the PCM is trying to compensate by lower engine load because of the input information coming from the mount.
Needless to say, understanding the modern motor mount needs just as much attention as the rest of the various systems in today’s cars. Keeping up with the technology also means you’ll be keeping up with the needs of your customers cars. 
