You've got classic symptoms of the fuel pump being driven from (only) the oil pressure switch.
HOPE that you have a failed relay. The contacts inside them can burn and then no current flows. Easy enough to swap relays from another circuit long enough to see if the pump primes.
HOWEVER, you could have a wire-harness problem, or even a failed ECM or ECM wire harness problem.
If the ECM or ECM harness failed, the relay doesn't get the trigger signal it needs to turn on the power to the fuel pump.
If the relay or relay harness fails, the relay doesn't turn on or there's an interruption in the power supply to the pump.
So the pump is driven by the oil pressure switch; but not until the engine cranks long enough to build oil pressure. While the engine is cranking, battery voltage is reduced, so the fuel pump starves for power.
Fix the relay problem first, then verify voltage and current to the pump with the engine running and alternator charging. Measure the voltage AS CLOSE TO THE PUMP AS YOU CAN ACCESS THE HARNESS. Excess voltage drop in the circuit could lower fuel pressure by starving the pump of power.
It's pretty typical that the pump is only supplied with 12-ish volts when the alternator is producing 14+ volts. GM uses under-sized wire in the pump circuit. Make sure the GROUND for the fuel pump has no more than 1 1/2 volts; again we'd like to see zero volts on the ground side, but skimpy wiring creates some resistance. The pump runs on about 3 volts less than system voltage; about 2 volts lost on the supply side, and about one volt lost on the ground. (Measured within about one foot of the fuel tank)
The voltage drop from harness corrosion INSIDE the tank is "invisible". I always replace the in-tank wire harness whenever I stuff a fuel pump into the tank.
HOPE that you have a failed relay. The contacts inside them can burn and then no current flows. Easy enough to swap relays from another circuit long enough to see if the pump primes.
HOWEVER, you could have a wire-harness problem, or even a failed ECM or ECM wire harness problem.
If the ECM or ECM harness failed, the relay doesn't get the trigger signal it needs to turn on the power to the fuel pump.
If the relay or relay harness fails, the relay doesn't turn on or there's an interruption in the power supply to the pump.
So the pump is driven by the oil pressure switch; but not until the engine cranks long enough to build oil pressure. While the engine is cranking, battery voltage is reduced, so the fuel pump starves for power.
Fix the relay problem first, then verify voltage and current to the pump with the engine running and alternator charging. Measure the voltage AS CLOSE TO THE PUMP AS YOU CAN ACCESS THE HARNESS. Excess voltage drop in the circuit could lower fuel pressure by starving the pump of power.
It's pretty typical that the pump is only supplied with 12-ish volts when the alternator is producing 14+ volts. GM uses under-sized wire in the pump circuit. Make sure the GROUND for the fuel pump has no more than 1 1/2 volts; again we'd like to see zero volts on the ground side, but skimpy wiring creates some resistance. The pump runs on about 3 volts less than system voltage; about 2 volts lost on the supply side, and about one volt lost on the ground. (Measured within about one foot of the fuel tank)
The voltage drop from harness corrosion INSIDE the tank is "invisible". I always replace the in-tank wire harness whenever I stuff a fuel pump into the tank.
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