1996 7.3 fires off, wont run

[motorhead440]

Apologies , I need to post this in the correct forum.

I'm helping my cousin get his 1996 F350 7.3 auto back up and running. It will fire off but only run for one or two seconds then it shuts off.

Changed the fuel filters, blew out the return line, replaced the fuel preheater. There was a lot of sludge when these parts were disassembled and cleaned. Truck started and ran fine after the previous maintenance for only about 20 miles then resumed the original symptoms.

Fuel seems to flow real well through the disconnected return however we havnt checked pressure at the shrader valve. Thinking about replacing the mechanical fuel pump and R&R the valve that switches between the fuel tanks.

Suggestions appreciated.