The big screw that holds the main fuel filter is supposed to have a metal seal ring just under the head of the bolt on the outside of the filter holder housing, and a rubber ring on the shaft of the bolt next to the filter. About 40 cents each at Performance, probably a buck each at the dealer.

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)