I had same issue with my 1993 6.5 turbo. Using this forum I learned that there are two fuel pumps. A primary, bolted to the lower frame that's below the drivers side.
A secondary fuel pump that is attached directly to the fuel injection system. Located on the intake manifold.
The primary pump provides fuel to the secondary. If the primary fails the secondary will still pump fuel
But, secondary is not designed for this and excessive fuel is used. Adding this strains this pump, if it fails you'll need at least $1500 to fix.
Two ways to check, each very easy. First, start truck, let run for 2 mins then stop. You should hear the primary pump "clicking". If not, this pump is not working.
Second, attach fuel pressure gauge to the "T" valve prime point. Start engine, open valve. If vacuum the primary pump isn't working.
Two points define what is likely wrong. Primary pump is bad or the OPS (oil pressure sensor) switch is bad.
With engine off:
To check primary pump, remove the plastic cover on the firewall behind the AC canister. You should see a fuse for pump. Using your fingers pop the fuse and wires off firewall. Then look to the right and you'll see terminals. Take the pump fuse/wire and touch fuse contact to terminal. If your hear "clicking" from under frame then the primary pump does work.
These engines have a weird sensor that is to shut down fuel pumping if in an accident. But these are common to fail.
O'Reilys has this part, Oil pressure sensor. Plus takes special socket to install, which they also have. Costs about $40, for both.
To replace the OPS get ready for sore fingers and lying on top of the motor.
Pop AL cover off and towards the very back of motor and just right of center you can see the OPS. It's down at back of intake manifold
Replace this and should get primary working again.
One point: with primary working you can hear it running.
1993 6.5L turbo.
1994 K2500 6.5L