Hi all !
1993 W250 5.9L Magnum.
Was running rough, stumbles at idle, dogs out/back fires with any throttle. New I had a burnt plug wire.
Did plugs, wire cap rotor. No improvement.
Was messing with timing and decided to squirt a little gas into
throttle plates and the engine smoothed right out.
Couldn't hear the fuel pump come on plus the filter is on the pump
in the tank, so I put a new pump and filter in. No improvement.
I had fuel at the shrader valve on the rail. Went to Rock Auto under fuel and they listed a Fuel pressure regulator. There isn't one on this truck at the rail, Only SS braided line from hard line on frame to fuel rail. I disconnected it and had a piece of clear hose same diameter as male line inserted in to SS quick connect and turned on key. Plenty of fuel volume for 2 secs until the relay shuts it off.
Put a guage on line and cycled the pump a couple times with key and could only get ~10 psi. During my troubleshoot I suspected voltage to pump, Ran a meter on line in and with truck running had 14.0 V.
Cleaned all grounds.
1) What should the Psi be ? ~ 40psi?
2) Should I measure psi at Shrader vale on rail?(dont' have an adapter)Can I pull valve out like a tire shrader vale and plumb to guage?
3) Could it be plugged injector(s) ? Should I replace all of them?

Any advice appreciated !