Did you do a compression test? If you have weak compression, squirt oil in the cylinders and do it again. If the compression comes up, the rings are probably weak, if not the compression is lost to the valves.

Good luck,

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Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles