I have a 1985 300D with a 617.952 (turbo) engine and a 1981 300D with a 617.912 (non-turbo) engine. The 1985 300D runs smooth through the entire rpm band. My 1981 shakes/vibrates a lot at idle. This car has 204,000 miles. A few months ago I changed the engine mounts. This helped to dampen, but the shaking and vibration is still significant at idle. Recently, I have adjusted the valves and performed a diesel purge, however, this seemed to make no difference. When I push down the pedal the engine shaking and vibration disappears. A few weeks ago I drove a 1980 300D with a 617.912 (non-turbo) engine. This engine ran as smooth as my turbo.

I have driven this car several hundred miles on the interstate at speeds between 70-80mph. The car does great on the highway and will achieve 27.5mpg.

Any ideas why my 1981 is such a bone rattler?

what is the idle speed on each? 100rpm can make a big difference in my 83. Next...how are the injectors?

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81 Mercedes 300SD 289k.......SOLD
82 Mercedes 300CD 252k......slow ride

82 mercedes 300 SD...mi Unknown
83 Mercedes 300D ????ksniff..gone too
84 Mercedes 300D 148k........SOLD
85 Mercedes 300TD 386k and holding some one elses project

I do not know that engne speed as my 1981 does not have a tach. The injectors........I do not know. Outside of the low idle shaking the engine runs great.

I would try bumping the idle up just a hair and see if fit settles down.If idle speed is below 600rpm it will shake like that.Use your ears and try to hear the speed of the smooth motor, duplicate that sound on the shaker.

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81 Mercedes 300SD 289k.......SOLD
82 Mercedes 300CD 252k......slow ride

82 mercedes 300 SD...mi Unknown
83 Mercedes 300D ????ksniff..gone too
84 Mercedes 300D 148k........SOLD
85 Mercedes 300TD 386k and holding some one elses project

When I turn the idle knob up all of the way the engine is pretty decent, however, I do not like to leave it there long because it takes a fair bit of foot to hold it still at a stop.

you should'nt have to turn it all the way up. check fuel lines for leaks, may be sucking air, have the injectors checked.Just my thoughts

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81 Mercedes 300SD 289k.......SOLD
82 Mercedes 300CD 252k......slow ride

82 mercedes 300 SD...mi Unknown
83 Mercedes 300D ????ksniff..gone too
84 Mercedes 300D 148k........SOLD
85 Mercedes 300TD 386k and holding some one elses project

I am 99.9% confident that it is not suching air. How do yo adjust the injectors?

If you are unsure of the injectors, and want to test them, they will need to be removed and taken to an injection shop for pop and balance testing. Be sure to install new heat shield discs when you put them back. You might want to check the timing chain stretch and valve adjustments before you go to all the hassle of the injectors. Perhaps even check the pump timing.

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Chris
64 190D R.I.P.
80 240D W/617 engine -for sale
82 240D -for sale