Your issue is an issue of fuel starvation.

Remove your shut off valve, like stewart says, and make sure it is not interfering with the fuel rack.

There is obviously an issue with the fuel rack in the pump.

when did this start? and how?

__________________
Pierre Hedary 1985 280SEL, 1970 111.026 in progress
Mercedes technical advice hotline: 407 765 2867
[servicing older Mercedes in the state of Florida with competence and passion].

From the standpoint of an honest MB technician, Vegetable oil is a threat to your diesel engine's mechanical integrity.