My 99 7.4l MPI has been giving me issues. Total rebuild, ran great then 2 weeks ago it was surging over 2k with a load. Put a fuel pressure regulator on it, key on engine off its 41psi. Up to ~1400 rpms it stays steady, then you can watch it drop to 0psi, engine slows down and pressure regains. So I thought it was starving for fuel. New 1/2 fuel line. Cleaned tank senders, still did it. Swapped tank pickups, still does it. Tried the relays off my other motor (running perfectly) and it still does it. So I wired in a 12v hot wire to pump, runs fine. Check all the wiring and there are no breaks. Swapped EVM's, new TPS and MAP and still doing it.
I'm at a loss. Any leads? Thanks.

Mike

Is the VST tank running dry?? fuel pickup or filter plugged/restricting fuel??, low pressure mechanical pump getting tired?? The filter on the high pressure pump dirty??