Introduction

The motors on W123 cars are supported in the front by a pair of rubber motor mounts. The mounts attach the motor to the frame of the car while buffering the motor to prevent excess vibration from being felt by the driver.

When these mounts begin to compress and soften, with age and with deterioration from exposure to fluid leaks, increased vibration will be felt in the car. In addition, the motor will ride lower in the engine compartment and, in time, will lower to the point where the accessory belts may rub against critical parts like the oil cooler lines. This can lead to a leak in the line, loss of oil, and eventually catastrophic engine failure.

So as you can see, the mounts play a critical role in the operation of a W123. Replace your worn mounts early, before trouble starts.

To remove your motor mounts you will need to raise, and support, your engine with a bottle jack. Before you can do this you will need to remove the bolts that hold the engine to the mounts. This is done from below the car. The bolts are allen head bolts; you will need an 8mm allen head socket. It should be a 1/2 inch drive socket as you will want a large wrench for leverage. These are often in very tightly.
  • To remove your motor mounts you will need to raise, and support, your engine with a bottle jack.

  • Before you can do this you will need to remove the bolts that hold the engine to the mounts. This is done from below the car.

  • The bolts are allen head bolts; you will need an 8mm allen head socket. It should be a 1/2 inch drive socket as you will want a large wrench for leverage. These are often in very tightly.

  • The bolt on each mount is located up inside a recessed area as shown.

  • Remove the bolt and set it aside.

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This allen head bolt is prone to stripping; i.e. the allen socket rounds out the center of the bolt so it cannot be backed out with a regular allen socket. If this happens to you try using a screw extractor. You may need to drill out the head of the bolt with a bit sized to match your extractor. Replace the bolt with a new one if this happens.
  • This allen head bolt is prone to stripping; i.e. the allen socket rounds out the center of the bolt so it cannot be backed out with a regular allen socket.

  • If this happens to you try using a screw extractor. You may need to drill out the head of the bolt with a bit sized to match your extractor.

  • Replace the bolt with a new one if this happens.

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Remove the lower 10mm nut on your engine shocks so the shock can move up as you jack up the engine. In order to loosen this nut you will need to keep the shaft on the bottom of the engine shock from turning with it.
  • Remove the lower 10mm nut on your engine shocks so the shock can move up as you jack up the engine.

  • In order to loosen this nut you will need to keep the shaft on the bottom of the engine shock from turning with it.

  • Just above the large metal washer on the shaft there is a flat area where a 7mm crescent wrench will fit on the shaft to hold it. It can be seen in this picture from above the car.

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Place your bottle jack under the oil pan and place a wide, thick piece of wood between the bottle jack and the oil pan to spread out the weight of the engine.
  • Place your bottle jack under the oil pan and place a wide, thick piece of wood between the bottle jack and the oil pan to spread out the weight of the engine.

  • Never directly jack up your engine from the oil pan as you may dent the pan. Always use a protective buffer like the wood block.

  • Jack up the engine a few inches. This provides easier access to the upper 6mm allen bolts on the mounts.

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Remove the two allen bolts that attach each motor mount to the frame of the car. They are 6mm allen head bolts. The outboard ones, those furthest towards the outside of the car (both sides of the car shown), are best accessed from above with a 6mm allen socket and a very long extension or several shorter extensions. The inboard ones, those closest to the engine, need to be removed from below the car with a 6mm allen key. These are difficult to see, much less take a picture of, but can be removed by feel fairly easily. They can be found almost directly below the metal arm that comes off the engine that attaches to the top of each mount.
  • Remove the two allen bolts that attach each motor mount to the frame of the car. They are 6mm allen head bolts.

  • The outboard ones, those furthest towards the outside of the car (both sides of the car shown), are best accessed from above with a 6mm allen socket and a very long extension or several shorter extensions.

  • The inboard ones, those closest to the engine, need to be removed from below the car with a 6mm allen key. These are difficult to see, much less take a picture of, but can be removed by feel fairly easily. They can be found almost directly below the metal arm that comes off the engine that attaches to the top of each mount.

  • Pictured are the bottoms of these two bolts. Inboard is shown highlighted in green, outboard in blue, to help show location from below the car.

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Jack up your engine a bit more to fully clear the motor mounts. Be sure to watch that you do not jack the engine up too high - watch that you do not crimp any heater lines or other items on the firewall behind the engine. Remove the mounts along with the heat shield.
  • Jack up your engine a bit more to fully clear the motor mounts. Be sure to watch that you do not jack the engine up too high - watch that you do not crimp any heater lines or other items on the firewall behind the engine.

  • Remove the mounts along with the heat shield.

  • Place the heat shield on top of the new mounts before re-installing them.

  • While you have your mounts out and the engine jacked up consider replacing your engine shocks if you feel they may be worn. These jobs are best done together instead of one at a time. Click here to see a guide on replacing the engine shocks on a non-turbo W123; the general concept applies to turbo cars though the shock mounts are different.

Before you jack up the engine be sure to disconnect the rubber turbo to air cleaner hose on those models with the big air cleaner or you will damage the hose.

Pat Sweeney - Reply

Conclusion

To reassemble your device, follow these instructions in reverse order. Torque all bolts to proper specifications.

12 other people completed this guide.

Nicolas Siemsen

Member since: 12/06/2013

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17 Comments

Well written guide. Very easy job to do, took no time at all. Highly recommend doing the engine struts at the same time. I didn't have any issues with stripped bolts, little tip to make sure you don't is to start the allen key socket by hand and then give it a nice couple of taps with a hammer to really seat it in the bolt.

Matt Smallwood - Reply

Matt, good points. Glad it went OK for you!

One other tip I've found has helped when doing other motor mount jobs, or any job that makes use of allen bolts, is to have an allen key/socket that fits very tightly in the head and that has a very flat tip so it sits as deeply in the head as possible. Too many allen sockets/keys are poorly made and aren't a good fit leading to increased chances of rounding out the inside of the allen bolt. I've taken to using a grinding wheel to flatten the tips of any all sockets that have a tapered tip.

Nicolas Siemsen -

Have to release the engine stop (part number: 123 240 03 65) to raise the engine and insert the new mounts.

Thomas Liu - Reply

Hello Thomas, thanks for the contribution.

However, that is untrue on the vast majority of W123's at least here in the US. The engine stop you mention is equipped only on very early W123's, and then only sporadically it seems. I've worked on a number of these from 1978 thru 1985 and none have been equipped with the engine stop.

From what I've been able to deduce from research, the cars that are most frequently equipped with these are early 240D's, pre-1980, and perhaps some gas engine cars which aren't very common here in the US.

So for the vast majority of people doing this job, they won't need to worry about the engine stop.

For those who do, they can refer to the factory manual on how to proceed and really they should because the manual notes a method for adjusting the engine placement on those cars equipped with an engine stop that is unique to those cars.

Nicolas Siemsen -

Thank you for the clarification. The 240D was made in January 1981. The manual mentioned the engine stop, but the picture was not clear where it was located. There was a drawing for a self made gauge, but not sure where it is used. Was able to remove the driver’s side engine mount and shock absorber but cannot install the new mounts. Almost destroyed the oil pan trying to lift the engine, but the chassis comes up with the engine. Finally installed the old parts back and took it to a Mercedes mechanic. He was the one who showed me the engine stop located at the back of oil pan. The area was dirty and the part was not readily visible. Lesson learned: clean the area first.

Thomas Liu - Reply

One of the bolt holding the driver’s side engine mount is directly under the bracket holding the shock absorber. Only access was from the bottom. Have to use a rigged up tool to work on it.

Option 1:

Metric Hex Insert Bit 1/4" Hex Shank, 6MM Hex Size, 1" Length - McMaster Carr part number: 7389A26

¼” Ratcheting box wrench - McMaster Carr part number:5163A11

Option 2:

Cut-off a portion of the long end of a 6MM L hex key, 1.25 inch long and a 6MM ratcheting box wrench. For some reason, a 6MM replaceable bit is no longer available.

Hex key – McMaster Carr part number: 5984A47

Tool to cut the Key – Dremel EZ Lock: EZ406-02

6MM Ratcheting box wrench - McMaster Carr part number: 5163A22

Due to the blind installation, almost destroyed the threads of the nutplate and the bolt. Used an 8MM x 1.25 tap and die to chase the threads in the nutplate and the threads on the bolt.

Mileage: 292,390

Thomas Liu - Reply

Thanks for the well written and pictured instruction. Just a little food for thought....so I just replaced my motor mounts, two months ago with these $20 meyle brand mounts, (since the OME mercedes ones are $150 each), but the driver side one has already failed. and i have the rough idle. This time I am going to try the 150 one and see if it will last longer....I guess you get what you pay for. I would also like to add that there is no fuel or oil leaking onto the mount,...when I replaced my motor mounts two months ago, at the same time, I replaced the front crank seal, had the timing chain replaced, replaced the oil filter housing gasket, oil pan gasket, I have replaced all fuel lines and the diesel fuel filters. I guess the cheap mounts dont hold up

Nightshade Acorn - Reply

Aftermarket parts can indeed have quality issues. Another option is to source Lemforder mounts. They make the ones that Mercedes sells under the Genuine brand. You should be able to get them for under $50.

Nicolas Siemsen -

Nicolas, I'm going to be changing motor mounts on a non-turbo 300D 1981. Worried about the "engine stop" T-bracket, or if my car has it. What year is your car? Is it Turbo or N/A?

Martin Mathews - Reply

The car this work was done on was a 1979 N/A car. It did not have the engine stop on the front of the engine, aka the t-bracket. I have never seen one that does. I think it was a pretty unusual thing and mostly seen on the 240Ds.

Nicolas Siemsen -

Thanks for the guide. Job was easy, went well. Hardest bolt to loosen was passenger side inboard mount bolt. Would recommend adding a step before jacking up engine. Take fan shroud clips off so the fan can rise with the engine and not hit shroud.

Martin Mathews - Reply

Martin, good point. Thanks!

Nicolas Siemsen -

Thank you!! The way the bolts are recessed I don't think I could have ever found them without your directions. Took me about 2.5 hours to replace both mounts, not a bad job for a guy with pretty much zero mechanical ability. Car is smooth as silk now, belts are clear from oil cooler line. (1984 300D) Thanks again.

David Malcomb - Reply

Awesome! Glad it went well for you. Now that you've done this job, you'll probably be bitten by the repair bug and want to tackle other jobs. Hopefully my other guides help with that.

Nicolas Siemsen -

What about mercedes w123 front motor t-mount replacement?

hjoab - Reply

I have not personally done that, so I have not created a guide for it.

The reason I have not done it is that the T-mount seems to be installed only on a small amount of these cars. None of the W123s I have worked on have it. I have never seen a 300D with one. Maybe some of the very early models had them? Otherwise I have only heard of the 240Ds having them.

In the meantime you can probably find a copy of the service manual PDF of those pages if you search around online.

Nicolas Siemsen -

I've detached the 8mm allen bolts and the shocks, but when I jack up from the oil pan the entire front end moves up. Do I have to keep going, or could something else be holding the engine down? How many inches should the front end go up before the engine starts to lift? I don't want to damage the oil pan.

PKE - Reply

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