I had the same problem with my 240D, tinkered with it for weeks.
Than just did one ‘LubroMolly Diesel Purge’, an excellent cleaning agent. I think it comes with instructions, you just need a mason jar and some clear tubing.
After that I have used an additive with every fill up, 1/2 pint of ‘Power Service’, mixed with a bit of 2cycle oil.
You learn all this, just cruising this great forum. These guys an gals are a trip and have an enormous knowledge.
You get her going, mine is just ticking away.

Good Luck
Wolfgang

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Wolfgang Marquardt
1982 240D, 270K
1983 Ford F100, 260K