Hello all,
I'm new to the forum, and it seems that there are indeed some very knowledgeable benz folks here. I bought an '81 300TDT last June and absolutely love the car. My question concerns the shaking/rocking back and forth that happens at idle. I have read about this issue being caused by weakening/wearing of the damper springs and bolt in the fuel pump, and thus causing a slight mistiming of fuel pulses to the injectors. I have also seen for sale an upgrade fuel pump rack damper bolt that is supposed to solve this.
I mentioned this to the guy I bought the car from, who is extremely knowledgeable (was a MB diesel tech for 36 years), and he said don't touch it. He said it only applies to some late 70's models. The upgrade rack damper bolts I have seen have been specifically advertised for 617 turbo engines.
Has anyone else dealt with this? Is it as easy as it sounds to replace this bolt, and will it really solve the problem? I am afraid that over time the car will shake itself apart, or at least create a lot more wear and tear than necessary.
I am mechanically competent. I understand the oil filter housing needs to come off to get to the bolt. Any advice anyone can offer as to go for it, or leave it alone, and why, would be helpful. Anyone out there do this fix before?

Thanks!

 

Are you sure the engine mounts are not blown?

 

As soon as I read the post, I thought the same thing, too. Check your engine mounts first. Cracked, damaged, sagging, etc. engine mounts need to be replaced. If that doesn't solve the problem, then you are probably going to need to look at something within the engine block. I would say misfire, but it's only at idle, correct? What happens at low RPM, high speed gentle acceleration?

 

Take an extrs good look at the Drivers Side Motor Mounts as the Power Steering Belt somt times cuts through the Oil Cooler Hose and that can take out your Engine.

 

This was my experience with shaking at idle after the Engine was warmed up.
Make sure you idle speed is correct. On mine it is 750-800 is normal (the info is on the Emission Sticker that also has the Valve Adjustment clearances).

If you Engine has the old style shiny Aluminum Bodied Hand Primer it is often the source of an Air Leak. Air leaks cause a rough idle.

I bought a new Rack Damper Bolt but it did not help much. After doing the below I concluded that the Rack Damper Bolt is designed to remove some shaking from an Engine that is running within spec. (That bolt was made to reduce the shake on a new engine; not to compensate for a bunch of worn and out of spec parts.)


The first 2 took away the bulk of the shaking.
Rebuilt my Injectors with $16 each brand X Spray Nozzles (This took away most of the shaking)

Doing a Valve Adjustement (took away some more shaking)

Changed Motor and Transmission mounts (took away some more vibration)

Found that I had a small air leak that showed up when the Engine was hot due to hard fuel inlet lines that did not seal even with the clamps tight. A few Dollars worth of 5/16" ID Hose fixed that. The Engne came with a New Style Hand Primer.

Finally was able to use the Rack Damper Bolt to tune out a little more shake.

 

Thanks for the advice! The idle speed is in spec, and I don't believe there is any air getting into the fuel system (although I'm not 100% sure). I will check out the motor mounts. The rubber fuel lines look good. The engine runs well overall, and the shaking disappears at higher rpm's. Sometimes it even idles pretty smoothly (very minimal shaking) but other times it it rocks back and forth a lot. I ran some injector cleaner (lucas brand) through the system and that seemed to help somewhat. I want to try some BG44k and see if that helps any more. I have heard great things about that product. I plan on doing a valve adjustment soon, I have some wrenches coming to me in the near future. Overall the engine runs strong and smoothly while driving. Any other suggestions are always welcome. Thanks again for the replies!

-Alex

 

The Stock Injector Spray Nozzles have a tiny hole drilled in the Tip and another larger hole drilled acrossed.
What I found on my set is that the both the tiny hole and the cross drilled hole were entirely plugged with carbon. Most of the Injectors I pulled at the Junk yard were in the same shape.

The issue with Valve Adjustment is that as the Valve Faces/Seats wear the Valve Lash Clearances get smaller. This changes when the open and close. Some forum members have had the clearances tight enough that they could not get the proper sized Feeler Gauge in at all.
While Injector Cleaner might have prevented the plugging I do not think Injector Cleaner will clean it out. Especially in reguards to the tiny hole in the tip as the cleaner would never get inside of it.
The holes are there for idle and low speed Fuel quanities. So that is why on mine it also ran OK at higher speeds but not at idle.