Check the suction side hoses from the standpipe in the engine compartment to the lift pump. If damp with fuel or original, I'd bet they are leaking and you are pulling air into the IP.

If you have the original red capped screw in type hand pump, replace it, it's likely shot. If you use it and it leaks fuel, definitely replace it, it's leaking air in as well.

Crack the big screw on top of the fuel filter and operate the hand pump -- if lots of air comes out, but never fuel only, you have an intake leak. If you can't operate the pump easily, like the filters are clogged with crap from bad fuel -- I would change them "on spec" -- your symptoms are typical of air leaks or clogged filters.

If you cannot purge the air out of the filters or the hand pump won't lift or return by itself (new type with internal spring), the tank screen in plugged with algae and you need to remove and replace it. Algicide in fuel treatment will kill any remaining in the tank.

Clogged screen is much more likely with low fuel by the way.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!