Have your mechanic re-torque the pressure valve holders. The copper seal can leak, resulting in rough idle -- I got my 300D cheap because #4 didn't fire at all at idle and #s 2 & 3 were weak. I replaced the seals, and found that #4 pressure valve holder was so loose I could unscrew it without the wrench!

What happens is that the leaking seal prevents pressure from holding in the injection line at low speed and low injection volume. The amount of fuel injected on the affected cylinder is low, causing rough idle.

The effect tends to disappear at higher speed, especially if the leak is minor.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!