Engine Idling - 1981 300D

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?