Sounds like the Lift-Pump to me....(yup, they do fail...)

Is the lift-pump delivering any pressure and any quantity...?

--Should be at least 7psi, but more like 10-15psi....

If the number 1 IP outlet/barrel/plunger were completely stuffed, it would still run on the other 4.
--It would obviously have a huge miss, but would start and run 'Reasonably' easily....

The main pump components, of elements, del. valves etc are pretty reliable, but can be damaged over time by use of Undried Veg-oil or poorly made BioDiesel that still contains moisture (Water)

Nah, Still points to Lift-pump from the description of fault IMHO....

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Alastair AKA H.C.II South Wales, U.K. based member

W123, 1985 300TD Wagon, 256K,
-Most recent M.B. purchase, Cost-a-plenty, Gulps BioDiesel extravagantly, and I love it like an old dog.

W114, 1975 280E Custard Yellow,
-Great above decks needs chassis welding--Really will do it this year....