That could happen because of corroded wires in the harness or a loose connection somewhere causing voltage drop across the wires. check the connections at the pump and trace the wires back as much as you can looking for breaks or corrosion, or loose connectors. if you can use a multimeter i'd suggest ohming out any wires you can that feed that circuit. I'm only speculating here but i'm thinking maybe the voltage/amperage spike is causing the pump to run properly. HOWEVER i could be totally wrong because i don't know for sure how the voltage is fed to the pump (if it's simply an on/off command or if it is PWM or what sent from the computer)

 

I think I won't get around dropping the tank again. It's just a stupid setup, neither the wires, nor the fuel lines are long enough to safely drop the tank and then unhook everything.
Very very frustrating for sure. Do you know if the wires go all the way through from the tank to the engine compartment, I mean factory.

 

The fact that it will start with a boost in voltage leads one to believe that the pump is weak (either due to a problem in the pump or with it's power supply) and needs the additional voltage provided by the boost from the charger to provide adequate pressure. As you have found, 50-52psi is not enough to get fuel into the cylinders with the stock SCFI spider (same holds true for the CPI spider).

A corroded connection could cause an increase of resistance on the pump circuit that could reduce voltage to the pump causing the symptoms. This could be confirmed by dropping the tank down a few inches and back probing the pump power & ground connections, monitoring the voltage when you key it on. You could also test the resistance on the ground wire to ground & the resistance on the +12V lead from the relay to the appropriate post at the relay as an alternative to back probing the connections. Both should be only a few ohms.

 

Every time I have needed to change a pump, I was able to jack up the rear, put the truck on stands and remove the left rear tire and remove the spare tire. After that lower the tank about 2 inches and then reach in from above the axle at the back of the tank and the wiring and fuel lines are very easy to see and work with.

 

I haven't been able to work on the jimmy in a few weeks, but today we tried to test some more.
-tested Volts at the plugs for the pump: purple to ground = 5V when ignition on
grey to ground = 12 V for a few seconds when
the ignition is on

Ground wires to pump are all connected directly to the frame.

Pressure does not go higher then 50psi with ignition on, an odd thing (maybe you can clarify) is that (maybe I never paid attention to it before) that the pressure will spike after the truck actually ran and is switched off again.
That sounds backwards to me, why does it spike after the engine is turned off? But even that wasn't happening on a consistent basis.

I'm loosing all my marbles here, what am I missing, or what should our next step be?