For W124 models with non-V8 engines, there is an easy method to check if the hydraulic engine mounts have collapsed. This factory document shows how to measure the mounts... if the distance is below spec, the mount is bad, and should be replaced. Again, this procedure only works on 124's with 4/5/6 cyl engines, not the V8.
However, no such procedure exists for V8 models (400E/500E/E420/E500). So, I kind of created my own procedure. While it's not necessarily going to be 100% accurate, it should at least give you a rough idea of the engine mount condition. Basically you measure the distance between the oil pan and the frame crossmember... if you don't have a telescoping gauge as shown in the photo below, slide various thicknesses of sockets into the space and measure the diameter of the largest that will still slip through the gap easily. I'd estimate that if you measure 20mm or less, your mounts are very likely in need of replacement.
Why not just replace the mounts if you're not sure? Short answer - time and money. The mountsare were about $200/pair for OEM before they went NLA (see below)... or, $420/pair $600/pair MSRP at the dealer as of Feb-2016. And the job is quite miserable... book time is approx 2-3 hours labor, but the job can sometimes requires double that time ($$$), as described in this thread. Some shops may charge 4-8 hours just because they hate the job so much. No point in shelling out for something you don't need, IMO...
The mounts in my E500 were toast after 15+ years, but the only symptom appeared in spring 2009 as I noticed a vibration at idle which hadn't been present before. The car otherwise ran fine, and there was no vibration at speed, only at idle. With new mounts installed, the idle vibration went away. Photos are below.
Additional photos at this link: Index of /images/M119/engine_mounts
UPDATE: Engine mounts for 124.034/.036 are MB part number 124-240-26-17. List price as of December 2024 is $256 each at the dealer, ~$200/ea from discount Rev Parts dealers. The OEM Lemforder aftermarket mounts went NLA around 2017, after that only low-quality, offshore-produced mounts were available for $25-$100 (BBR, ÜRO, Febi, Rein). If fixing a beater to flip, sure, install the cheap mounts. If it's a car you care about, shell out for the OE Genuine (made in Germany) mounts, while you can... I'm confident that once inventory is depleted, these will go NLA. The 140 mount p/n 140-240-22-17 is an alternate number that is physically interchangeable, the same caveats apply with aftermarket vs OE.
However, no such procedure exists for V8 models (400E/500E/E420/E500). So, I kind of created my own procedure. While it's not necessarily going to be 100% accurate, it should at least give you a rough idea of the engine mount condition. Basically you measure the distance between the oil pan and the frame crossmember... if you don't have a telescoping gauge as shown in the photo below, slide various thicknesses of sockets into the space and measure the diameter of the largest that will still slip through the gap easily. I'd estimate that if you measure 20mm or less, your mounts are very likely in need of replacement.
Why not just replace the mounts if you're not sure? Short answer - time and money. The mounts
The mounts in my E500 were toast after 15+ years, but the only symptom appeared in spring 2009 as I noticed a vibration at idle which hadn't been present before. The car otherwise ran fine, and there was no vibration at speed, only at idle. With new mounts installed, the idle vibration went away. Photos are below.
Additional photos at this link: Index of /images/M119/engine_mounts
UPDATE: Engine mounts for 124.034/.036 are MB part number 124-240-26-17. List price as of December 2024 is $256 each at the dealer, ~$200/ea from discount Rev Parts dealers. The OEM Lemforder aftermarket mounts went NLA around 2017, after that only low-quality, offshore-produced mounts were available for $25-$100 (BBR, ÜRO, Febi, Rein). If fixing a beater to flip, sure, install the cheap mounts. If it's a car you care about, shell out for the OE Genuine (made in Germany) mounts, while you can... I'm confident that once inventory is depleted, these will go NLA. The 140 mount p/n 140-240-22-17 is an alternate number that is physically interchangeable, the same caveats apply with aftermarket vs OE.
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