Motor Mount Age of Replacement and While You Are In There

[Jlaa]

Hi gang,

One drawback to eliminating primary order vibration is that it makes you (me) so sensitive to vibrations that you (me) notice secondary and tertiary order vibrations.

As a recap, I recently replaced my tires which were slightly flat spotted, which were causing primary order vibrations at speed.

Replacing and road force balancing the tires exposed secondary micro-vibrations that were felt through the steering wheel at 65mph+. These micro-vibrations could be seen with the eye as the steering wheel would micro-vibrate.

Re-roadforce balancing the fronts wheels (0.25 oz difference on 2nd balance) and adjusting the front alignment (toe, especially RF toe was way off at 0.50* toe in) eliminated those secondary micro-vibes and the steering wheel is now smooth.

Solving this matter, however, has now exposed tertiary order nano-vibrations which can be felt through the floorboards, and most discernible at 68mph. Lower speeds don’t manifest these tertiary vibes unless *perhaps* when the car is under load (steep uphill).

I am going to re-roadforce balance the rear wheels but I suspect maybe the issue is not wheels, but is related to aging (but not totally deflated or completely worn) motor/trans mounts.

The car is already smoother than a vast majority of the 30 year old w124s but I am in search of the ne plus ultra of smoothness.

The two driveshaft flex discs and center support bearing were replaced in 2017, 9000 miles ago. However, my records show that a previous owner replaced the motor mounts 12 years ago and 20K miles ago.

My question is - how likely is it that the TIME (12 years) has contributed to some deterioration of the motor mounts?

As well, anyone have any thoughts on “while you are in there” type activities to conduct during motor/trans mount replacement?

Thanks

 

[Kyiv]

Mate, have you seen Dave's post on checking out engine mounts condition by measuring the distance between front cross-member and, of I remember correctly, upper oil pan?

Post #1

HOW TO: Check engine mount condition | M119 Engine

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...

www.500eboard.co

 

[gsxr]

Have you measured the distance between oil pan & engine crossmember, to determine if the mounts have partially collapsed? (Kiev was posting as I typed, lol). I'm curious if they have sagged a bit over 12 years.

Also, at hot idle, does the engine feel glass smooth, or can you feel micro-vibes from the driver seat? One of my cars is dead smooth, another not quite, both have low-mile mounts that are not sagged/collapsed (I'd have to check the age though).

"While you are in there" stuff would be anything related to the starter, steering box, P/S hose along the frame rail, knock sensors, AC compressor, or lower engine harness. Access to those items is easier with the engine mounts removed.

The transmission mount (aka rear engine mount) can sort of be checked by looking at the air gap on top, which should be relatively small. A low-mile mount should not be collapsed though. I think there's a photo or two on W124performance somewhere.

Engine mounts for the 124.034/.036 are MB part # 124-240-26-17. MSRP $266 each, ~$200/ea from discount dealers, get OE/Genuine only... all aftermarket are Chinese now, no bueno:

1978-1999 Mercedes-Benz Engine Mount 124-240-26-17 | MB OEM Parts

1978-1999 Mercedes-Benz part # 124-240-26-17 - Engine Mount

www.mboemparts.com

 

[Jlaa]

Mate, have you seen Dave's post on checking out engine mounts condition by measuring the distance between front cross-member and, of I remember correctly, upper oil pan?

Post #1

HOW TO: Check engine mount condition | M119 Engine

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...

www.500eboard.co

Have you measured the distance between oil pan & engine crossmember, to determine if the mounts have partially collapsed?

Also, at hot idle, does the engine feel glass smooth, or can you feel micro-vibes from the driver seat? One of my cars is dead smooth, another not quite, both have low-mile mounts that are not sagged/collapsed (I'd have to check the age though).

"While you are in there" stuff would be anything related to the starter, steering box, P/S hose along the frame rail, knock sensors, AC compressor, or lower engine harness. Access to those items is easier with the engine mounts removed.

Ohhhh thanks @kiev! I remember seeing @gsxr’s guide to measuring the oil pan to cross member gap and I was really dreading having to crawl under the car on jackstands and trying to measure the gap with a tape measure.

However I just reread the guide and realized that I can use various sockets as a “feeler gauge!” What a clever trick! I will measure this weekend.

@gsxr it is interesting to note that your mounts were toast at 15 years of age. I recall my car’s hot idle being glass smooth (I wasn’t noticing any vibes at idle) …. Is it worth replacing the knock sensors and AC compressor while there or “leave it alone if not problematic?” My lower harness I replaced 5 years ago. My lower PS hose —- I need to double check if I replaced that.

 

[gerryvz]

Motor mounts should be replaced between 60 and 80,000 miles. By 80K they are very definitely in need of replacement.

 

[gsxr]

@Jlaa, I would not mess with the knock sensors or AC compressor unless you have a reason to suspect either of them. Or at least wait for the 6.xL upgrade.

Remember there are 2 "lower" PS hoses, the S-shaped hose from engine to frame rail, and a straight section along the frame rail hidden by a heat shield. The latter is MUCH easier to access with the exhaust crossover pipe and engine mount/bracket removed.

Keep in mind that if your idle is glass smooth, new mounts are unlikely to make any difference at speed... you could give it a try, but it will be ±$600 in parts plus several hours labor to find out.

 

[Jlaa]

Ok, griminess aside, I measure 24.13mm between the upper oil pan and the front crossmember. I think the existing motor mounts are probably fine. I will re-balance the rear wheels and see what happens.

 

[Jlaa]

The transmission mount (aka rear engine mount) can sort of be checked by looking at the air gap on top, which should be relatively small. A low-mile mount should not be collapsed though. I think there's a photo or two on W124performance somewhere.

Thanks! These are the photos on your website. Where may I ask is the transmission mount in the photo of the car? It is difficult to see exactly….



I spoke w/ @jhodg5ck today - he mentioned that an 11 year old trans mount is most definitely time for replacement…… hmmmm

I should also mention that Jono indicated that something has recently changed with MB sourced trans mounts and that recent mounts are not very long lasting —- he has taken to filling the voids with new trans mounts w some sort of 50 or 60 shore hardness material.

The mount on my shelf is from 2015

 

[Jlaa]

Ah ok, I see how the trans “rear motor mount” attaches now from trans pics on w124perf. I’m dense - I still can’t make heads or tails out of the “Note the size of the gap here” pic.

 

[gsxr]

I need to edit the annotation on the old photo. The "size of the gap" picture shows a non-V8 transmission mount.

The V8 mount, as you can see in the next photos, is turned sideways vs the non-V8. The same concept applies though.

Note that when installed in the car, it is normal for a good mount to have the air gap somewhere in between the new/unloaded position, and the collapsed mount pictured.

Here is what you would see on a V8, with the large silver aluminum mounting plate removed: