I HAD A similar leak in my 90 350sdl. it turned out the injector seal was missing. it is a little steel ring thing. looks like a button with one hole sort of. it is something which could fall off the end of the injector i believe. if missing will cause leaks or also affect the injector spray pattern in the combustion chamber causing excessive diesel knock. on my car all six had been left out. it was running poorly, smoking some and had such a loud knock that you would swear something mechanical was about to break. if this is missing it could be a simple fix... which usually is the case with benzes although sometimes hard to track down.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.