Low fuel pressure (limp mode 40psi) under load

[daniel4]

Hello I am struggling looking for ideas how to fix my low fuel pressure under load problem- my best guess is the fuel pump control module is not receiving the fuel pressure voltage from the fuel line pressure sensor- an electrical short somewhere. So the computer puts the truck into fuel pressure limp mode at 40 psi.

Car: 2011 gmc sierra 1500 sle 5.3 LC9 (has flex fuel option)

Problem: Fuel pressure drops to 40psi. Car starts at 55-58psi then after 10-30+ seconds the fuel pressure drops and remains at 40psi. Throws p0171 & p0300. I think it is because not enough fuel so the computer starts tweaking the short term fuel trims and they get to about 30%- which is when the codes appear

 

What Ive done so far:

• New fuel pump ( DELPHI FG1057)
• New fuel pump control module (GM version & reprogrammed)
• New fuel line pressure sensor & gasket & wiring harness connector

 

Other things related to code but not fuel pressure

• Reset fuel alcohol content levels
• New intake manifold gasket
• New throttle body gasket
• Cleaned MAF sensor
• Cleaned injectors
• New injector gaskets
• Multiple passing smoke leak tests
• New bank 1 & bank 2 downstream o2 sensors

 

Troubleshooting and how I arrived at best guess an electrical short

• Fuel pump appears to be fine- starts at proper pressure. I jumped the FPCM (fuel pump control module) to force the pump to always run. I put a load on the truck while in park and the fuel pressure stayed at 90psi for way longer than 30 seconds (normally would drop down to 40 psi)
  • (pressure verified at both fuel rail shrader valve via obd2 port)
• Jumped the fuel pressure sensor by connecting fuel pressure sensor 5v voltage in to fuel pressure sensor out. I get ~111 psi and again the fuel pressure does not drop down to 40psi under load while truck is in parked
  • So this tells me
    • electrical circuitry between engine, fuel pump, and fuel pump control module is intact
    • pump can output 111psi fine
    • fpcm can output 111psi fine
• Visually inspected ground screws near fuel system. They seem fine?
• Tested resistance around the pump, fuel rail pressure sensor, & fpcm
• I probed the FPCM pin where the voltage from the fuel line pressure sensor voltage should come in. I get near 0.
  • So I think something is shorting out and causing fuel line pressure sensor voltage to not reach the fpcm. The FPCM then does not see voltage for fuel pressure and puts the truck into limp mode at 40psi. 

 

The problem is I am at a loss for what to test next. Ive checked common engine grounds and they appear fine? Any suggestions, ideas? Very frustrating to figure this out

[CamGTP]

Seeing fuel pressure at 40psi is normal though. If the demand for the fuel is not there the fuel pump control module will reduce fuel pressure. I've been tuning these trucks for a long time now and seeing a range of 40-60psi of fuel pressure based on demand is totally normal. The injector flow rate table in the calibration has the flow rate data from basically 2 bar all the way to 5 bar fuel pressure.

 

 

If your fuel trims are maxing out at 30%, I wouldn't be looking at a fuel pump or control module first. I would want to look at what the computer is saying for alcohol content in the fuel. These flex fuel trucks run a virtual flex fuel system, meaning there is no actual sensor to tell the computer this data. It takes it's information from the o2 sensors. Sometimes the data gets skewed the truck reads it incorrectly. So if your alcohol/ethanol content in the scanner is reading like 40% alcohol content but you are running E10 rated fuel, that is going to cause big problems. 

[daniel4]

3 hours ago, CamGTP said:

Seeing fuel pressure at 40psi is normal though. If the demand for the fuel is not there the fuel pump control module will reduce fuel pressure. I've been tuning these trucks for a long time now and seeing a range of 40-60psi of fuel pressure based on demand is totally normal. The injector flow rate table in the calibration has the flow rate data from basically 2 bar all the way to 5 bar fuel pressure.

 

 

If your fuel trims are maxing out at 30%, I wouldn't be looking at a fuel pump or control module first. I would want to look at what the computer is saying for alcohol content in the fuel. These flex fuel trucks run a virtual flex fuel system, meaning there is no actual sensor to tell the computer this data. It takes it's information from the o2 sensors. Sometimes the data gets skewed the truck reads it incorrectly. So if your alcohol/ethanol content in the scanner is reading like 40% alcohol content but you are running E10 rated fuel, that is going to cause big problems. 

Thanks I appreciate the response. My understanding is ~43.5 psi at idle is normal. I am pretty sure ~40 psi under load is not normal. Many on the internet say it is common to see 55-58psi under load. Is it possible to never see over 43.5psi? I only see 55-58psi at startup, then never again

 

My fuel pressure commonly reads at 290-304 kpa while driving under a load- even when I drive it hard.

I have had the alcohol content reset twice over the past 6 months. (Most recently ~10 days ago) I think the last I checked it was around 10%? I will look again. I am only running 87 octane fuel

 

Oddly ive really only seen p0171 (bank 1), pretty sure no bank 2 codes over the past year ive been trying to fix this

 

Is it possible the upstream o2 sensors also need to be replaced? 2011 with 192k miles- likely the factory upstream sensors.

What about checking some part of the flex fuel system?

 

I thought the fpcm controls the fuel pressure to the pump, and the fpcm is informed by the fuel line pressure sensor voltage. 

This also wouldn't explain the fpcm harness pin out not getting the voltage from the fuel line pressure sensor.