Hi everybody, my brother purchased a 1991 560SEL that is making a tapping noise from the #2 cylinder on the right side of the engine. Its also missing on that cylinder. The spark plug is black and all the others look well. A compression test shows the cylinder at 155 psi. I took the injector out and it is slightly cruddy but not obviously bad. I am going to replace it tomorrow regardless. Do you guys have any suggestions on what to look for next. I am kind of stumped.

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1979 240D with a 617 turbo diesel.