My mother bequeathed her well-maintained 84 300D (253K) to me. It's been park and unused for over two years. I had a Mercedes specialist in CA get it in running condition for the 1600 mile trip to Houston. The car ran very well with lots of power running 75 mph most of the way (sometimes 90). After two tankfuls, I noticed the engine would shake wildly at idle. I also noticed that I only got 20 mpg. Put in 6 oz of Lucas for every tankful from then on.

What was done to it in CA: New tires, battery, air filter, alignment, engine and tranny oil change.

What I have done to it in Houston:
1) Valve adjustment (Dieselgiant procedure). All valves were tight with 2 exhaust and 2 intake almost w/o clearance. Noticeable improvement but still rough idle. Wish I had this done in CA but car was running well then.
2) Two cans of diesel purge (Dieselgiant procedure). Significant improvement in power. Idled well for a while but after driving for 20+ miles, started shaking again at idle. MPG increased to 23.
3) Adjusted idle but no help up or down so I put it back where it was originally

In order to determine whether the injectors are faulty, I cracked open 1/4 turn the injector supply at each injector. 1,2,4 did not further deteriorate the rough idle. What is interesting and not expected was that cracking open 3 and 5 actually improved the idle??? (or did it reduce the shaking because it also stopped contributing power and therefore "balanced" the engine??) I'm suspecting underperforming fuel injectors or bad fuel delivery (IP perhaps?).

Another symptom, I notice is a constant knocking sound towards the front. I stethoscoped the vacuum pump and don't hear it there. It seems to be coming from 1 or 2 cylinder (upper). Knocking not noticeable when 3 and 5 injectors cracked. As an aside, I have a 87 SDL that idles smoothly with little vibration. I definitely have a sick 300D. Any ideas guys?