Not sure if this quite fits, but when I bought my wagon it had a few oil leaks. Seller said most of it was caused by a bad gasket between the injection pump and motor (= costly fix). First thing I did when I took ownership of the car was change the oil. Guess what? The oil drain plug was loose. Screwed it in tight and that stopped 90 percent of the leaks.

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'81 300TD turbo <<sold 12/05 to another diesel nut