Low voltage to fuel pump 99’ burb P0171, 0174

[nathanisafatman]

Hey y’all. New to the forum.
I’m having a voltage supply problem to my fuel pump that I cannot for the life of me find the solution to.
THis is causing P1071 and P0174 lean bank codes.
99’ burb k1500 4x4. 158k miles. Viper security system installed by previous owner.
So far, I have replaced the FPR, fuel pump, fuel pump relay, MAF and battery.
Refreshed the battery to body ground, battery to block ground and fuel pump ground above the rear diff.
Voltage at the fuel pump relay terminal tests fine at battery voltage, but back probing at the fuel pump harness, I’m only getting 10 volts at prime. My grounds at the harness are all within spec.
I have tested all fuses in cab and engine bay and all are good.
Fuel pressure specs are: 60 PSI KEY ON ENGINE OFF, 50 PSI ENGINE ON. PRESSURE RISES WITH THROTTLE BLIPS, ALL THE WAY UP TO 60 PSI. WHEN TURNING ENGINE OFF, PSI JUMPS TO 62 PSI, AND HOLDS STEADILY WITHOUT DROPPING RIGHT AWAY AND HOLDS PRESSURE FOR 8-10 MINUTES.
I just cannot figure out where to go next so any wisdom would be greatly appreciated

 

[RawbDidIt]

Sounds to me like it isn't the fuel pump. Have you checked the pressure past the filter to ensure that isn't the problem? The only other thing in the fuel system it can be is the injectors, so I'd move to cheaper fixes if it were me. O2 sensors can cause the code as well as a vacuum or intake leak. Personally I'd check for the vacuum leak first, then replace the intake gasket, then replace the O2 sensors. You can always take it to a shop and have them run a diagnostic on it.

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[nathanisafatman]

Rawbdidit, I forgot to mention I did replace the fuel filter as well.
I’ve also done a smoke test and found no vacuum leaks. The plenum gasket seal was replaced when I replaced the FPR.
My upstream 02 sensors look to be original, but my down streams have been replaced.

 

[RawbDidIt]

Well ****. Wonder if the injectors aren't atomizing the fuel... one more cheap thing to check before you go down that road, how's the air filter, and does it happen to be a K&N?

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[nathanisafatman]

I haven’t replaced the air filter but I did spray it out with compressed air. It’s not a k&n.

 

[RawbDidIt]

It was a long shot anyway, odds are you won't throw a lean code even without a filter. Are there any symptoms when you're driving around?

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[east302]

That 10V at the pump is probably an issue. Was that measured by grounding the meter to the chassis or using the ground in the harness?




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[nathanisafatman]

East302 the 10 volts was measured with a lead to chassis ground.
I did take the burb out today (cranked up fine with no hesitation) and when pulling on to a busy road it started to hesitate badly and backfired a few times but didn't die. I quickly pulled it into the next parking lot over. As I gave it gas to get into the parking lot it continued to bog down but not die. I parked it and let it, turned the key off and let it sit for about a minute. Tried cranking it once, and it didn't start. Tried again and it fired right up. Drove it home with no problems.
I'm starting to guess it's both of my upstream 02 sensors??

 

[east302]

This is a long one, but here is the diagnostic table for P0171. At a glance, it looks to lead to the fuel system diagnostic tables. If it had an episode of not starting at all, it’s either fuel, spark or air that’s missing or intermittent. I wouldn’t really lean to an oxygen sensor for that symptom. Your low voltage at the pump would be my first guess.

Hope this helps.

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[Ken K]

You indicated a loss of voltage to the fuel pump. Let's do a voltage drop after inspecting the fuel pump connector, chassis connector (If any), and the relay. The "Prime" feature to run a fuel pump during testing, should be indicated on the fox / relay center lid. It is usually right next to the fuel pump relay. Using a fused jumper wire to positive, when placed on the "Prime" connector, the fuel pump comes on. Most techs, working against the clock, will simple hook a headlight bulb to the fuel pumps connector, under the back end. A headlight pulls about 4 amps, while a fuel pump runs about 7.0 - 7.5 amps. Build should be bright. If not, you have a bad B+ wire from the relay or ground to frame to battery issue. Voltage drop will work this way on the ground as well. Test the relay is next. Remove the relay and install 4 jumper wires about 5" long, with the correct terminal ends, and plug them into the relay in the correct locations. A DMM set to B+, connect the meter leads to the relays terminal #30 and #87. Have someone turn the key on for the two seconds prime the PCM provides, and take a reading. The best would be between 0.50 volts - 0.03 volts. The lower the better. Repeat several times to say "Catch It" with a high reading above 1.0 volt or more. The relay is bad if you get that one time high reading. If the relay is test good, plug a "Tee" pin or proper terminal (Side cutters at bone yard) and follow the positive wire to the pump and re-test. Should have same results.
The fuel pump. like most automotive circuits, are series circuits. The component (Fuel Pump) will consume all of the voltage in the circuit...So real close to battery. Across the pump, back-probed, with it running, will get the correct reading as current has to be flowing thru the circuit in order to check for loss of voltage. It is quick, easy, and does not lye. Every fuse, relay, connector or switch will have a small amount of voltage loss. This is voltage drop...plain and simple. If the leads are backward, you get the same reading, just the meter shows a (-) negative symbol on the screen. Hope this helps.
Radio shack sold "Meter Lead Wire" on spools, so I made 4 different lengths with alligator clips soldered on. This soft coated wire makes it easier to work with. My longest jumper is 14 foot and my go to for fuel pumps.