When I first got my 87 300D I had trouble with plugging filters. One of the things I did to help was to connect the suction of my external fuel pump/filter to the supply line from the tank and put the discharge back into the fuel tank. I ran it like that for about an hour. I think it helped. Who knows.

-Jim

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1995 S350D, Green with black leather interior.
Bought January 2008 w/ 233,xxx miles.
I did 22,000 miles during the first year of ownership.