76 300D W115 engine vibration
 

I have a mid RPM engine vibration in my 76 300D that has me completely baffled. It is like a harmonic vibration that happens going up to speed and then back down through the RPM range coming to a stop. I have read every post I could find for clues. This is what I have done so far.

Replaced engine mounts with new original MB parts.
Replaced tranny mount.
Replaced fan clutch (old one had a lot of play in it)

When it hits that certain RPM range the vibration can be heard throughout the car and felt in the gear shift lever. When sitting still the vibration is still there when you rev the engine which tells me it is not in the drive line or transmission.

Any clues to what I am missing?
Thanks for your help.

Has anything ever been done to the fuel system? New injectors, filters, primer pump, tank cleaned, fuel pressure relief valve stretched...

Ever seen a valve adjustment?

New air filter?

Has the driveshaft ever been off the car? The drive lines are sensitive to miss alignment causing vibration...

OK
 

If a mistake was made while changing the transmission mount, you could have this issue.
Please note the stiffening bracket and bolt in the following picture.
This bracket and bolt are critical, as they reduce movement and add strength to the transmission mount.
http://i21.photobucket.com/albums/b2...2999af02-1.jpg


If the front sub-frame mounts have failed, it will shift around making random (mild to horrible extreme noise is possible) body contact and exaggerating any engine movement.
http://i21.photobucket.com/albums/b2...ps81d50bc7.jpg


Some of the W115 chassis have at least one engine shock absorbor, if it or the bushings fail, expect excess engine movement and noise.
http://i21.photobucket.com/albums/b2...ps36acab2a.jpg


I understand you replaced the injector nozzles.
However I will note that the injectors in my daughters 1976 300D have been removed for cleaning (body and nozzle) several times, this has been due to sludge apparently trapped in the injection pump from previous infestation.
My repeated use of Diesel Purge, BioBor, StarTron, and vigorous manual fuel system cleaning has at this point eliminated the contamination.
http://i21.photobucket.com/albums/b2...psa9b1ad09.jpg

Here is the thread about my daughters car, with many links to procedures done.
http://www.peachparts.com/shopforum/diesel-discussion/321775-welcome-fleet-1976-w115-114-300d-dark-green.html

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everything done except valve adjustment. Very smooth except for those mid range rpm's.
Good suggestion though.
Thanks

I do not recall a stiffening bracket at all when I replaced the mount. Is that a standard original part?

I have not replaced the sub frame mounts. They are pretty sad looking but I didn't think this would isolate an engine vibration.

The single engine shock is shot but I thought this was only for idle shaking. The car idles very smooth.

Thanks for the suggestions.

^^^If I were you I would listen to whunter's advice. If you don't have a copy of the W115 FSM there is mention of this early extra stiffening bracket and the procedure for the adjustment of the transmission mount in the W123 FSM (which is more widely available).

Have you checked the front vibration damper,AKA the harmonic balancer. That dampener is capable of a lot of problems if the primary or big bolt gets a tad loose. Also the smaller studs holding the pulleys to the dampener can get loose and or crack off. I would recommend that with the engine running you really watch the motion down there at the very lower part of the front of the engine to see if any side motion is evident. You just don't want that assembly to get loose or it will cost you an engine.

Answer
 

Yes, the stiffening bracket is a standard original part.

Sub frame mounts on the W115 are important.

The single engine shock is important for more reasons, due in part to the odd Sub frame mounts on the W115.

.

There is no stiffening bracket on my W115. Not sure if it was removed by previous owner'(s) or what? The mount simply has two smaller bolts connected to the cross brace and one large bolt connecting it to the transmission.

I am leaning to think that it is the subframe mounts causing the resonating vibration noise. I see that the motor mounts are connected to the subframe and that could cause a metal to metal connection to the body. I do feel the vibration through the steering wheel as well.

At certain rpms the car is extremely smooth and quiet and then as you accelerate or decelerate you suddenly feel and hear that low bear grumbling sound.

My engine shock mount seems to be ok but the shock itself has no resistance so if I can find a new one I will replace.

Thanks again for the help!
John