This is good, you endlessly hurting down those niggly little things that can make owning an old car frustrating .

Back to your loss of power & fuel economy : your OM 603 (?) engine might be slightly different from the old 6 cylinder OM617's I fool with so I don't want to give useless advice so post up some clear, well lit pictures of the turbo and any hoses, pipes, banjo fittings and so on connected to it or the entire intake manifold, this will tell the collective here where to make the next search .

I just recently discovered a bad waste gate diaphragm (overlooked by everyone else who's looked at or touched my battered Coupe) and in teaching my self how to diagnose and repair it, I discovered the valve proper can get so coked up it remains stuck partially open, this too will reduce the boost in RPM application and total boost amount, once I figured out what to test and how, I made rapid progress, looking at your thread here I'm quite certain you'll learn and get this car running strongly again too .
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__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better