My 300D vibrates noisily like a steam shovel at idle. It has a newly
rebuilt engine, and the motor mounts were allegedly changed at the
time of the rebuild. I have had this high vibration at idle before on
other 240/300Ds, but have always been able to solve the problem with
new motor mounts.
I have been able to mitigate the shaking somewhat by increasing the
idle, but only by increasing the idle to a level higher than normal.
Any suggestions will be appreciated.
I have the same problem with my 1982 300 TDT with a newly
remanufactured 617 turbo aspirated 5 cylinder 3 liter diesel engine.
In discussions with Mike, at Metric Motors in Canoga Park, CA, the
shop that remanufactured the engine and Gus, at Pacific Fuel Injection
in South San Francisco, we have determined that the problem is the
mysterious harmonic vibration that builds between the oscillation of
the "rack" inside the injection pump and the engine itself.
On the injection pump on the 617 engines there is an adjustment bolt
with a lock nut on it on the rear of the pump. This provides the
ability to vary the pressure on the part inside the pump which
oscillates at idle, causing the vibration that feels like the engine
is "working against itself." It's a powerful shake. I am not using
that car right now and won't until I get this repaired because the
shake is bad enough that it is going to for sure damage something and
also shake the car apart if I ignore it.
It is only at idle. Going down the road everything is fine until you
take your foot off the accelerator and then especially when you step
on the brake like to stop at a light. Then the shaking at idle
commences and there seems little to do to stop it other than stepping
on the throttle again. As soon as I increase the engine speed above
idle, the condition ceases, but that is not always possible to do, for
example, when you are stepping on the brake to come to a stop you
would have to shift to neutral and also step lightly on the
accelerator at the same time, which is not always possible and is a
dangerous distraction and, in general, a nuisance.
So why have I not mentioned a solution yet? Well, if you are lucky
the "rack adjustment" bolt on the back of the injection pump will
solve the issue, but, according to Gus, who is a middle aged German
guy who has been rebuilding Mercedes injection pumps for over 30 years
and is know throughout the West Coast as the preferred source of
rebuilt pumps, the entire issue is also a mystery to Mercedes Benz
themselves. He says that the rack adjustment bolt itself has gone
through three redesigns and that it still does not always solve the
problem. He says that sometimes cracking open the injection lines at
the pump until they leak every so slightly sometimes can solve the
issue by slightly changing the injection rhythm enough to cancel out
the harmonic vibration.
I haven't tried cracking a line yet, but I did find a couple which
were slightly loose and even one that appeared to be leaking a tiny
bit. I tightened those two lines, but it had little or know effect on
the harmonic vibration that is about to shake my car to pieces!
I have tried adjusting the rack damper bolt a few times, but so far to
no success. I am going to try it again today. I got a thinner lock
nut so I could turn the bolt in further, but that has not helped so
Gus recommends adjusting the bolt as follows:
1. Loosen the lock nut. If you take the bolt out, there is an "O"
ring on the bolt inboard of the lock nut on the bolt you need to be
2. Turn the bolt out at least 3 turns. I took mine out entirely to
check it to make sure it was not defective or broken. There is a
spring loaded "pin" which comes out of the end of the bolt and it is
that pin that presses against a "lever" (according to Gus, I have not
had an injection pump apart myself) inside the pump. When you are
putting the bolt back in, you can feel it when you are turning the
bolt in by hand when it contacts the lever inside the pump. This is
where you begin to vary the pressure against that lever by turning in
3. Warm up the engine thoroughly! It is very important, according to
Gus and other mechanics I have spoken to, to warm up the engine.
Then, start to tighten the bolt, turning it gradually about a quarter
of a turn at a time.
4. When the engine smooths out, stop adjusting and tighten the lock
nut against the body of the pump and go for a test drive.
I have not had much success with this. The adjusting bolt does not
seem to do much regarding my engine's vibration - it's a nasty shake.
Mike at Metric, the remanufacturer suggests varying the timing
slightly, but Gus laughed about that and didn't think that was the
problem. My engine and pump are still under warranty, and Gus has
asked me to remove the pump from the engine and send it to him and he
will put it on his bench and if there are any problems he will send me
another pump. I think he is going to send me a different pump anyway
in hopes that it will be just different enough from the one I have now
that it won't set up the harmonic vibration.
"It is all a mystery" Gus repeatedly says.
Why don't you give it a try if your pump has such a bolt and let me
know the outcome?
I am going to try adjusting the bolt on my pump again today and see if
I can make any progress, and if not, I will try and get that pump off
later this week. On my 617 engine I have to remove the oil pump
housing, which is a pain in the ass, before I can get the pump out, so
I am really trying the other suggestions in hopes that they will
suddenly solve the issue.
I had this problem about 15 years ago before the engine was rebuilt
and the mechanics at my Mercedes dealer in Seattle recommended that I
replace the bolt and I did and it solved the problem.
Don't be surprised if your mechanic has not even heard of the rack
adjustment bolt. I spoken to several mechanics I think of as very
expert who have never dealt with this problem and did not even know of
the existence of the rack adjustment bolt.
I posted inquiries about this on this list a few times recently and
got zero replies.
I should add that my engine is about 2 years from the remanufacture
and has run splendidly until about a month ago when this vibration
appeared. I had to drive to Albuquerque a few weeks ago and I drove
the car the thousand miles to Albuquerque at regular highway speeds
and it ran smoothly and did great on the highway, but every time I had
to stop or do any city driving I was afraid the car wasn't going to
make it because of that damn vibration.
I ended up buying a BMW 330ci in Santa Fe that I was looking at on the
web before I left and had the Mercedes wagon shipped home on a car
I have about 23,000 miles on the rebuild. It gets the 33 mpg highway
that the car was rated at when new and has good power.
Thanks for this very interesting information. I was not aware of the
adjustment bolt. I do not think my vibration is nearly as bad as
yours, but, am going to do this adjustment and try to correct it, as
simply increasing the idle is not the answer.
I just got in from trying to adjust the rack damper bolt again. It's
slightly better, but still unacceptable. I want to drive around the
southern end of the Sierra and do some photography in a remote grove
of Giant Sequoia trees tomorrow, but I can't drive my BMW because the
clearance is too low for the last few miles of dirt road, and I don't
want to drive my 4Runner on the rest of the trip because it beats you
up compared to my BMW or Benz. The TDT would be the perfect car for
that expedition, but it still hammers like hell at idle, especially in
reverse, so I can't take it. I will stay home instead and try and get
the pump off and sent for repair/replacement at Gus' shop.
I tried cracking the lines a bit, but that just caused that cylinder
to quit firing and had no effect on the vibration.
It's the only bolt sticking out of the rear of the pump. On my pump
the bolt has a hex head that fits a 12 mm wrench and there is a lock
nut that tightens up against the body of the pump that fits a 14 mm
wrench. It is the only such double-looking bolt I have seen on the
car. It may be closer to the top than the bottom. On my engine it is
not far forward of the oil filter housing, which must be removed to
allow enough room to slide the injection pump out of the engine.
I can post a picture of it later today if you need more help in
finding the bolt.
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