Engine Shaking at Idle (W123 300D)

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 far.

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 aware of.
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 the bolt.
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 transport truck.

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.

I should say specifically that you have to have the engine running when you adjust the bolt in case it was not obvious in my earlier post.

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.

Is the adjustment screw you are referring to the one on top of the rear of the injector pump?

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.