daniel4

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.

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