Motor mounts are almost always problems on unrestored older cars for two simple reasons: they're rubber and they eventually wear out, and replacement is not straightforward if you don't know what you're doing.

As the primary care physician of many an older W116, W123 series or W126 series car, I have had the opportunity to change many sets of motor mounts. The simple design of the mounting system in these cars makes motor mount replacement somewhat easy,  given you manage to avoid denting your oil pan or stripping any Allen bolts.  The following article was intended for diesel  W116, W123 and W126 cars, but the lessons here can be applied to gasoline variants of the same bodies. (Note,  we know that motors are always electric and engines are internal combustion, but we’ll use the traditional term for this specific component.)

A new engine mount on the left, next to a worn mount. The line shows the height difference between the two due to compression over time. The old mount also has long since lost its cushioning resilience.

When Do You Need New Motor Mounts?

Every six months, try to inspect your motor mounts. If they are smashed flat, cracking, have oil seeping in and out of them, or one side is lower than another, replace them. If you have a heavy idle vibration that decreases with engine speed, replacement is probably in the cards. If you have a 280SE or 300SE that makes a groaning sound under hard acceleration, your motor mounts may also be at fault.

Tools of the Trade
Before you start any of the work, make sure you have the necessary tools. The tools you will need for this job are as follows:

• 7mm wrench
• 10mm socket wrench
• Small ratchet
• 6mm Allen socket
• About 24 inches of 3/8th-inch ratchet extension
• Half-inch drive ratchet or breaker bar
• Short extension, half-inch drive
• 8mm Allen socket [or a 10mm Allen for pre-1981 models]
• Pipe, if needed for additional leverage
• Awl or pick
• Impact driver
• Hammer
• Jack and block of wood
• Jack stands

 
Under the Car...
Once you have supported your vehicle safely on jack stands, you can crawl underneath. Towards the driver’s side you will see a recess for the 8mm motor mount bolt under the motor mount arm, and indirectly behind it will be a 10mm nut and rubber mounting for the engine shock. On non-turbo models, a central mounting is installed just past the lower oil pan. It is secured by a pair of 13mm bolts and by a 19mm bolt through the central cross member. It should be replaced as part of this job [replacement is straightforward, so we will not discuss it]. On the passenger side, you will see a lone mounting hole for the right side motor mount. Look out for a shock on this side if you have a turbo or 6-cylinder car.

Study the distance from the frame to the 8mm bolt head. If this bolt head is closer to the frame on one side, this is where the vibration is originating, as it indicates collapse of the rubber portion of the mount.

Start by loosening the engine shock. This requires a 7mm wrench to hold the shaft of the shock, and a 10mm wrench to undo the nut. Once this is done, undo the pair of 13mm bolts [and just the bolts, not the nuts!] for the central mounting. Then, using your breaker bar and 8mm Allen, undo those motor mount bolts. It helps to use an impact driver here, and to use an awl to clean out the hole in the Allen bolt. Often, these are filled with debris, so it is critical you clean it out. Once removed, the Allen bolts should be examined for damage to the bolt head and threads. I usually just install new bolts from M-B, and it pays next time you have to go in!

Now, disconnect the battery and detach the intake accordion tube, exhaust mounting,  fan shroud and the throttle linkage so that you don’t damage these items. Now support the oil pan using the jack and wooden block, and slowly raise the engine. As you raise the engine, look at the motor mount arms. Once you have reached a 2-3 inch clearance between the arms and the mount, you should have sufficient workspace.
 
The Main Event
Now that the engine is elevated, remove the motor mount heat shields. Do not forget to re-install them, as they are critical for motor mount longevity. You can now access the 6mm bolts on each side of the motor mount. These are, in order from easiest to hardest: The outer left, outer right, inner right and inner left. Here is the combination of tricks I use to get to each one, but make sure the Allen fittings are clean first!

• The outer left: I use all of my extensions to get to this one. This an easy one to get out – just try not to strip it.
• The outer right: I use a short extension, about an inch over the motor mount arm, to get clearance. The small ratchet is best here, so you don’t hit the exhaust.
• The inner right: I use the same tool for the outer right, and I go from the top of the car. Go underneath to fit your socket if necessary, and make sure it goes all the way in. With the engine all the way up, a swivel joint on a quarter-inch extension should not be needed.
• The inner left: This is the dreaded one. However, with a small quarter-inch drive 6mm Allen and small ratchet with a small head, you should have sufficient room. Once it is loose, try to unscrew the bolt by hand, so you can feel the threads and see if they are okay.

Loosening the Motor Mounts -- Pictorial

The driver’s side motor mount hole recess in the frame. You can also see the shock mount.

Passenger side motor mount bolt. Note its proximity to the starter and to the lower control arm bushing

The central motor mount (with four bolts). The two outermost bolts attach it to the bottom of the oil pan. This item was not installed on turbodiesels. It may be missing on your engine, but it is critical to a smooth and stable idle.

The driver’s side motor mount and heat shield. This shows the relationship between the motor mount, the heat shield and the mounting arm.

With the fan shroud, exhaust bracket, throttle linkage and motor mount bolts disconnected, the engine can be carefully jacked up. Note the use of jack stands under the car.

The passenger side motor mount from above.

Driver’s side motor mount from above, showing its location relative to the rest of the engine.

It is critical that the top of the motor mount arm be aligned with the center of the mount. There is a recess in the mount (arrow) in which the arm actually sits.

The engine shock mount showing the sequence of rubber mountings for it. A 7mm combination wrench (upper arrow) holds the bolt secure while so that a socket can be used to loosen the nut (lower arrow).

Replacing the Mounts

Slide your old mounts out and install the new ones with one of the 6mm Allen bolts in its hole. These bolts are going to be used to set the motor mount in place, and should be hand-threaded. Install the easiest side first, and then, using your 6mm Allen, install the inner [harder bolts]. This can be tricky, but cleaning your bolts, threads and surface really, really helps. It really irks me when I see only one motor mount bolt installed.

Once your bolts are hand-threaded, tighten them down to about 15-20 lb.-ft. of torque. In other words, only as hard as needed, since you don’t want to strip one of these.  Next, place the heat shields on the motor mounts and lower the engine most of the way down.

Go back under the car and check the mounting holes. The jack can be used to center the oil pan and the engine, and the holes can be centered with a quarter-inch extension. Once you get one bolt threaded, install it about half the way and then go to the other side. I usually start on the driver’s side and go the passenger side, but you can do what is easiest. To line up the other side, I use the quarter-inch extension again. Once you get the bolt hole and the motor mount lined up, let the jack down. The weight of the engine will lock the two together so they stay aligned. Hand-thread the bolt, if possible, and tighten both of them to 35-40 lb.-ft.

Remember to reattach the engine shock, central mount, and the ancillaries you disconnected in the engine bay.
 
So Which Motor Mounts Do I install?

Motor mounts are not very tricky, but getting the right ones can be confusing.  I always stress the need for original equipment parts here; the aftermarket stuff does not feel right or last as long. Mercedes has several rubber hardness designations for their mounts. The common ones are S1 (very hard), S4 (intermediate), S7 (soft),  and S11 (very soft).The designations are stamped on the mount itself.  Always use your vehicle identification number (VIN) when ordering parts from your Mercedes dealer. The chart at right shows which  cars take which mount. All mounts are similar in shape, with the S1 being the exception.

Ideally, you should call your dealer and order the parts based on your VIN, but you can use this chart to see if the component you received is suitable. Also, cars that have vibration issues can be given harder mounts to deal with the problem. As our cars age, they take on unique characteristics, and as engines accrue miles and stories, they may need some extra magic to be as smooth as they were before.

In the event a heat shield is missing, they also are still available and definite should be replaced since they’ll extend the longevity of your motor mounts substantially.
 

What do I do if things don't go by the book?

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