I was looking at some pics of the 1991 Mercedes 300D fuel injection pump. After the 3 flange nuts are loosened you can turn the adjustment to change your fuel injection pump timing.

If there is not some sort line already scribed from the block to the fuel injection pump flange (so you can return it back to where it was) you can make one. Simply try advancing your fuel injection pump timing a tiny bit and snug down the flange nut bolts and see what happens.

Fuel quality can also case nailing. I got an amount of bad fuel many years back that did that.

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84 300D, 82 Volvo 244Gl Diesel

It is on my to do list to verify timing. Thing is it could be so many things. What I know is that one time I put in a used injector and it was quiet. But yet when I did it again, it knocked. This is what spurred the question of “is it me?

It has also been thousands of miles with the knock since I owned it. PO said it was a lifter. On certain occasions, or maybe almost always, it has been next to impossible to trace to a specific injector. This why I thought it was lifters. Only after putting in another remanufactured injector have I really found the ability for knocking to effectively be on or off on #4. But with a different, quiet injector. Which again points to something in doing or something else.

IP seems quiet with stethoscope.

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
2008 ML320 CDI (199k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)