5.7 vortec long crank

[slow_c1500]

Ok so the fuel pressure is 54psi for the first 2 seconds when the key is turned to the on position. Then it clicks, and the pressure slowly drops down.

Yep, you have fueling issue(s).

Is 54 too low? Or is the pressure not supposed to drop down after the 2 seconds where the pump cuts off?

 

[Erik the Awful]

I should also mention that the truck usually starts right up in the morning, after it’s been sitting overnight. It’s just during the day when it hasn’t been sitting as long, it cranks for a while and starts.

I think you have more than one problem. This is a pretty good sign of a vacuum leak. When cold, your engine has an enriched mixture that can hide a vacuum leak. When warmed up it no longer enrichens the mixture and makes it harder to start.

 

[PlayingWithTBI]

Is 54 too low? Or is the pressure not supposed to drop down after the 2 seconds where the pump cuts off?

Both, pressure s/b ~60 psi with KOEO and it shouldn't leak down for ~30 seconds or more.

 

[slow_c1500]

Yep, you have fuelin

Both, pressure s/b ~60 psi with KOEO and it shouldn't leak down for ~30 seconds or more.

Ok. I ordered a fuel pump relay and I will see if that solves the fueling issue.

 

[Schurkey]

I would have tested the voltage as close to the pump as practical, with the pump running. This is typically at the rearmost wiring connector before the harness goes up and over the gas tank.

We know the fuel pump relay "works", because you have 54 psi during prime. Therefore, the ECM and the wire harness from the ECM that turns the relay on and off also works.

We don't know how much voltage drop the relay has, or how much VD is in the rest of the fuel pump wiring harness--which is why a voltage check as far rearward as you have access would have been nice.

And I would test the accuracy of that fuel pressure gauge, because Hazzard Fraught often sells "tool-shaped objects" rather than actual tools. It's possible you have the 60-ish psi, but the gauge isn't telling the truth.

 

[Caman96]

Ok. I ordered a fuel pump relay and I will see if that solves the fueling issue.

This may or may not be correct, but I was told(and I’ve done this) to put your finger on or listen closely to relay while someone turns key on. If you feel or hear “click” the relay is working. But like @Schurkey points out, you already know it’s working.

 

[slow_c1500]

I would have tested the voltage as close to the pump as practical, with the pump running. This is typically at the rearmost wiring connector before the harness goes up and over the gas tank.

We know the fuel pump relay "works", because you have 54 psi during prime. Therefore, the ECM and the wire harness from the ECM that turns the relay on and off also works.

We don't know how much voltage drop the relay has, or how much VD is in the rest of the fuel pump wiring harness--which is why a voltage check as far rearward as you have access would have been nice.

And I would test the accuracy of that fuel pressure gauge, because Hazzard Fraught often sells "tool-shaped objects" rather than actual tools. It's possible you have the 60-ish psi, but the gauge isn't telling the truth.

How can I test the gauge to make sure it’s telling the correct psi?

I can get a multimeter to test that voltage. Where would I touch the black and red wires to, and should I do it with engine running?

And if fueling happens to not be the problem, would a vacuum leak be the next most likely issue?

 

[Caman96]

Haven’t you determined fuel pressure is already low? If you don’t trust gauge, borrow another from auto parts store.

 

[Caman96]

I can get a multimeter to test that voltage. Where would I touch the black and red wires to, and should I do it with engine running?

Unplug fuel pump connector on frame, feel top of frame and you’ll find it just as it’s going to tank. Probe unplugged harness while someone just turns key on. Should come on for like 3 seconds and then stop. I checked with test light first.
Grey-power to fuel pump
Purple-Sender fuel level
BLK/Wht- Ground to sender
Black- w/eyelet grounds to frame.
IIRC

 

[Schurkey]

Unplug fuel pump connector on frame, feel top of frame and you’ll find it just as it’s going to tank. Probe unplugged harness while someone just turns key on. Should come on for like 3 seconds and then stop. I checked with test light first.
Grey-power to fuel pump
Purple-Sender fuel level
BLK/Wht- Ground to sender
Black- w/eyelet grounds to frame.
IIRC

Testing with the connector unplugged will not find corroded wires or poor connections. It'll find BROKEN wires or DEAD connections, but we don't have that in this case since the pump does run albeit with lower-than-expected pressure which MIGHT be due to lower-than-expected voltage, including perhaps high resistance on the ground side.

If there's no current (amperage) flow, resistance in the circuit won't lower the voltage. He needs to know if there's resistance in the circuit--power side, and ground side.

Therefore the pump MUST be running to properly test the system voltage. Best if the engine is running, so the alternator is charging.

When it's me, I use a matching connector pair with a short section of wires in-between. Plug the male connector into the female, plug the female connector into the male, use the "special tool" connectors-and-wires to pierce and probe with the volt meter so the actual vehicle wiring isn't damaged. They also make specific tools to back-probe the connections without damaging the wires. That'd work, too.