Fuel Pressure Issues

[DMac94]

* The power input to the 2 switches in parallel. In other words, do we have a flaky fusible link?
Or is it the feed from the battery -to- the fusible link?

I have not yet bypassed this fuse, but it would be worth a shot for diagnostics. I did look at the fuse/connectors, and it all looked clean.

* Or on the far side of the fuel pump, there is only 1 ground wire from pump motor to frame.
No doubt that you have already checked/cleaned this frame ground back to new condition?

Yes, I pulled the ground off and even checked the top of the frame rail with a mirror. It's clean.

NOTE: IF this proves to be a tough intermittent, then you could use a couple of cheap LEDs
in the cab and wire them to the output of the fuel pump relay as well as the output of the
oil pressure switch. I would expect with Key On the fuel pump relay LED will come on for 2
seconds and go out.

You then start cranking the engine, and the fuel pump relay LED comes right back on,
(thanks to Reference pulses coming into the ECM) engine fires, and shortly afterwards the
oil pressure switch LED comes on also. And they should both stay on for the duration of the trip.

Of course you could add a third LED, connected right at the fuel pump connector, to monitor
the wiring between these switches and the power feed at the pump itself . IF all 3 stay on
solid and you lose fuel pressure (assuming a perfect ground on the far side of the pump) then
we now know that the pump itself is flaky (perpetrator) since power never flickered in this scenario.

I will likely build a little harness to plug in at the sending unit connector to implement this and check voltage/amperage while driving. Anything to avoid dropping the tank. I feel like at this point after still having the issue with the FPR jumpered and taking the FPR (and PCM signal) out of the equation, I am going to work from the tank forward verifying quality power supply.

Apologies for the length, but just wanted to give you the '94 FSM diagram for your viewing
pleasure. (And if you haven't already done so, you really want to download the '94 FSMs
for your vehicle: '88+ FSM links)

No problem, I really appreciate the detail

 

[Schurkey]

I will add though that I can stomp on the gas while cruising, and the fuel pressure stays absolutely steady. I would think this would require more fuel than starting from a standstill, but maybe not?
The truck currently has 1/4 tank of fuel

FILL THE TANK, test drive, see what happens.

Maybe the sudden acceleration is causing the fuel to slosh away from the pump--broken/missing baffles in the tank, or perhaps the fuel level is lower than 1/4 tank, but the guage is "optimistic".

 

[scott2093]

Any updates?