low fuel pressure after replacing injectors

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sircrisp

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I have a 99 yukon 5.7 vortec and just replaced the oil pressure sensor, distributor, plugs, wires, intake manifold gaskets, and injectors with new ac delco mpfi injectors. The truck ran fine before the work but now will not start. Sometimes it will idle but the second I touch the throttle it dies, but the majority of the time it just runs for about 5 seconds or just won't start at all.

I turn the key on, hear the fuel pump prime for a couple seconds and shut off. With a fuel gauge attached at the rail it only goes to 10-15 psi when it primes and then drops to 0 when the pump shuts off. I thought it was my fuel pump so I then replaced it with a new delphi pump and replaced the fuel filter but the issue persists.

I'm starting to lean towards the new injector assembly being bad but don't know if it would warrant such a large drop in pressure. Before I take the upper intake off I wanted to seek advice here.
 

sircrisp

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FP regulator? I'm not familiar with 5.7 though.
That is what I'm thinking because it is part of the new injector assembly, just not sure if it could be the source of a 50 psi drop from what it's supposed to be.
 

Pro439

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You got lucky and got a covid part sounds like they never set the fuel psi regulator. If you’re old 1 is good I would think you could use it. If you’re looking for something a little better that is flowed and checked Wester’s Garage sells them. He can also do programming
 

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RichLo

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Also maybe try pulling the plugs to check if 1 is dripping, could be a stuck injector
 

sircrisp

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I jumpered the fuel pump relay to run the pump and held the schrader valve open and got a nice steady stream of fuel so I am guessing that the FP regulator is stuck open or closed (i dont know which way these work) and is returning most of the fuel to the tank or intake instead of restricting it so it isn't building pressure.
 

Schurkey

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I am guessing that the FP regulator is stuck open or closed (i dont know which way these work) and is returning most of the fuel to the tank or intake instead of restricting it so it isn't building pressure.
Stuck open. That seems most likely. I suppose leaking injectors would have the same symptoms except you'd fill the cylinder(s) with fuel.

Another possibility is some sort of fuel leak into the intake manifold.

Is the distributor installed properly? Wouldn't affect fuel pressure, but could affect starting/running.
 

mattillac

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I'd (very carefully) clamp the return line (at the flexible section under the drivers side), then momentarily run the fuel pump to see if the pressure increases.

If the pressure didn't increase with the return line clamped, I'd remove the fuel filter and check fuel pressure there.

Verify the pump is good before condemning the spider.
 

Papikev252

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I'd (very carefully) clamp the return line (at the flexible section under the drivers side), then momentarily run the fuel pump to see if the pressure increases.

If the pressure didn't increase with the return line clamped, I'd remove the fuel filter and check fuel pressure there.

Verify the pump is good before condemning the spider.
What if it does increase? I have the same issues but no long/hard start. Only low fuel pressure while running and after prime is done.
 

east302

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If the return line is blocked and pressure builds, then the regulator was the source of the leak.

If doesn’t build pressure, then suspect a leaking injector, faulty pump or pump check valve. You’d need to pressurize and then isolate the supply line to determine if it’s the pump or injector. A ball valve and some hose installed in place of the fuel filter would be one way to do that.
 
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