You seem to have done a good job of narrowing down the problem.

Now its a matter of being a bad fuel pump, a pretty big hole in the fuel line between the pre-filter and the fuel pump, or whatever part drives the fuel pump is broken.

I haven't done a MB fuel pump yet, so I don't know how integrated it is into the injection pump assembly - and if it is a replaceable sub-assembly.

One thing I'd suggest you do before any substantial fuel pump disassembly, and that is to replace the fuel line temporarily between the prefilter and the fuel pump. Or take it off and pressure test it somehow. I'd hate to see you replace a component only to find out its a bad fuel line.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles