Low fuel pressure-is my test accurate

[Reega]

89 GMC K1500 350 700R4

My truck starts but dies almost instantly. If I give it about half the pedal, it’ll kinda run, but sounds like it’s doing everything it the world to try and die and does as such shortly.

I hooked up my Actron gauge at the back of the TBI- it’s not on the return line I checked- and cranked and it fired and died and showed low or no PSI.

I jumped the pump at the ALDL and got like 4psi. Is jumping the pump an accurate way to test? I jumped from the courtesy like fuse to the pump. Each time showed around 4 psi. Just wanted to make sure before I drop the tank. Thanks!!

 

[Schurkey]

That's a valid pressure test--but--you need to verify that the filter isn't plugged, and that the pressure regulator isn't faulty. I'd start with the filter, and see what happens. If the pressure is still low, you could block the return line to see what the pressure is with the system dead-headed.

It's not a valid VOLUME test, but there's no concern for volume if the pressure isn't there. If you had appropriate pressure but still have fuel-starvation issues, you'd want to run the pressure test with the vehicle being driven under load--uphill with heavy throttle, or pulling a hefty trailer, for example. Or mimic heavy load by using the bypass button on the pressure tester to bleed off some volume and see if the pump can maintain pressure.





For the record, I won't replace a fuel pump without measuring the voltage near the pump, or without measuring the amperage draw of the pump. If there's poor voltage to the pump motor, the pressure will be low.

Whatever the battery voltage is--typically 14+ volts with the engine running--the pump better have within a volt-and-a-half of the battery voltage. GM uses under-sized wire to power the pump, and there's often one or even 1.5 volts difference, lower at the pump versus at the battery. Any more than 1.5 volts and you have a wiring defect--corroded wires, burnt contacts in the fuel pump relay, that sort of thing. The ground side of the pump may have another volt of loss, again due to undersized wiring. The pump therefore only gets 11.5 volts when the alternator is putting out 14 volts, but the loss has to be measured separately on the + side and the - side.

If you replace the pump, be sure to get an in-tank wire harness. The harness in the gas tank is known to corrode also, and you can't measure the loss in that harness because there's no external access points. You can often see the corrosion, though, once the pump is out of the tank. Here's a photo of the new, and the old harness connector from the in-tank harness for the fuel pump in my Lumina.

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[Reega]

Wow, thanks man!

 

[Reega]

Been awhile, but wanted to update. I suspect the pump was still good, but awhile back I had to replace the sending unit due to a blown brake line behind the fuel tank. The lines wouldn’t break loose and eventually the sending unit gave way. Anyways, used the provided lines with the sending unit and wouldn’t ya know, they swelled and split. Put a new Delphi in anyways.

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