One diagnostic is to monitor intake boost pressure from inside the cabin. I put a T in the plastic tube from the intake manifold and ran a nylon vac tube thru the rubber grommet to a gage in the cabin to monitor while driving. Some have made that permanent (gage where cigaretter tray sits). At the same time, unscrew the intake banjo bolt and clean it out (as mentioned).

You don't know what you will find. I did above when a newly swapped engine wouldn't let my 1985 go faster than 5 mph, which was strange since it ran fine when I tested it on the ground. The vac gage showed ~20"Hg vacuum in the intake as I tried to accelerate. As above, while normal for a gas engine, a diesel should have almost no intake vacuum other than the slight air filter drop. Caveat - some M-B diesels do have a throttle plate (post 13). I popped off the plastic elbow and found the wad of paper towels I had stuffed to keep debris out of the turbo inlet. My punishment for re-assembling after dark. Ran great w/ no air restriction.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans