I have been busy the last couple of weeks and am just now getting cought up with my thread reading. I have a 91 300D which is almost the same as yours. As for your problem, First, the only way a diesel can run rich is not getting enough air. If you are getting too much fuel, the engine will just run faster. First, what you need to do is unhook the vacuum lines from EGR, Wastgate and the air flap in the intake. In fact, you could just remove the part of the intake system that runs over the valve cover. If it is still acting like it is rich (black smoke) then make sure the exhaust is not pluged. You could unhook the exhaust pipe from the turbo and again check for the black smoke. If it is still there, then the only other possibility is a timing issue. Ususally if the cam is too far out of time the compression will be low and it won't start at all. However if the injection pump is out of time you could have black smoke due to the fuel getting into the cyl at the wrong time and geting incomplete combustion. I know for a fact that these cars will run OK, not great but OK with no boost. After you get it running smooth, then you can worry about all the other things.
Gook luck

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84 500 SEL (307,xxx miles)