I believe he mentioned already doing that with no improvement...
The only other thing I can think of is an air leak in the fuel system somewhere. This is definitely a fixable problem..

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'79 300D, Ivory "Gunther" ......going
'81 300SD, Metallic Blue Silver, The Grey Ghost.....going
'87 300D Turbo ...gone
'05 CDI.... gone

Ops ! Oh yes, so he did...sorry

Air getting in supply maybe, but after that 'tank' test it would exclude the tank outlet and lines up to where he married in the hoses for the tank.
I would now try another lift pump..easy to do.

...or did he try that too!


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