6.6L Duramax LML Fuel Return Testing

Marlin Technician Oregon Posted   Latest  
2015 GMC Sierra 3500 HD SLT 6.6L (8 LML) 6-spd (1000) — 1GT423E83FF154705
Crank / No Start
Excessive Fuel Return

This one stalled while driving, and will not build enough fuel pressure to be measured by the Fuel Pressure Sensor. The priming pump will build pressure, but the pressure quickly bleeds off. It pulls 2 inHg while cranking. No debris was found in a sample collected from the return line, at the connection to the hose which is external to the engine.

I disconnected the outlet hose from the Fuel Injector Return Line Constant Pressure Regulator (Retention Valve). This valve is designed to maintain a minimum operating pressure of 43 PSI in the injector return lines at the injectors. While cranking, a very slight amount of flow exited the valve, which means that one or more injector or the Back Fill Valve are leaking slightly (not nearly enough to keep in from starting, based on my experience). From the disconnected hose a much larger volume flowed, and fuel flows rapidly from this hose while operating the priming pump. This makes me suspect a failure of the HP pump (as that is the only source then connected to the hose), and that the "overflow valve" (not sure what GM calls it, I cannot find any mention of it in SI) is allowing excessive flow or flow at too low pressure. Or, is this return flow actually controlled by FPR1 (Fuel Pressure Regulator 1)?

It seems to me that it should be a valid and safe test to block the return line from the pump (steel pipe), and run the injector return (rubber hose) into a container, then attempt to start the engine. This would eliminate the FPR2 (Fuel Pressure Regulator 2, mounted on the LH fuel rail) as the fault, if the engine runs. Removing FPR2 and inspecting its screen for debris (specified in SI) is potentially valid, but this requires several hours to do and interferes with further testing.

Any helpful comments and suggestions are requested. I hope that this can be made into an informative thread that shares more efficient diagnostic steps than what are specified by GM. Please help to fill in the gaps in information which I left open, correct any errors, and add any relevant information. GM's diagnostic process is convoluted, inefficient, and missing information. Some of the system description is incorrect. This is the first time that I have dug far into an LML, its fuel system is quite different in some aspects as compared to previous versions, and I found very little fuel system info for it either here or on iATN. Access to components such as FPR2 and its hose nipple is quite time consuming, though SI just refers to it as a casual process.

Robert Owner

Marlin, see attached . . . Regards, Bob

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Marlin Technician

Thanks, Bob, I appreciate the effort. However, I find no benefit in the file relating to my particular situation. #1 is incorrect regarding the location of FPR2; it is mounted to the front of the rail, under the EGR cooler. The rest is …, but just basic, info.

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John Technical Support Specialist
New York

Hi Marlin, Pull the FPR1 out of the pump and inspect it for metal, The pumps have a problem of coming apart and going through the fuel system​.​If you find metal you are changing all the fuel system,GM has a bulliten for this, the number is 16-NA-102

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