Mustang mtl20 not keeping primed

Hi I have a 2004 mustang mtl 20 not able to keep the fuel system primed. I have cleaned out the outlet elbow at bottom of tank, replaced entire line from elbow to transfer pump, replaced transfer pump and wiring to it, replaced injectors, even tried bypassing fuel water separator bowl with still no success. I am leaning toward injection pump replacement. Any ides or things I may be over looking. Engine is an isuzu 4jg1t