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2007 GMC 5.3L Truck P0171 and P0174 - Starts on Second Try, Fuel Pressure Drops After Shutdown

Model: 2007 GMC Truck Fault Code: P0171 Posted: 2009-07-14 13:48

I've searched the forums extensively but can't find a solution for my 2007 GMC 5.3L V8 truck. The vehicle will not crank successfully on the first attempt—typically taking about 5 seconds—and starts smoothly on the second try. Once running, it operates normally with no idle issues or stalling. However, I'm seeing OBD2 fault codes P0171 (System Too Lean Bank 1) and P0174 (System Too Lean Bank 2). The fuel pressure drops completely when the engine is turned off, only building up after starting. This behavior suggests a potential issue with the fuel pump check valve or low fuel pressure during cranking. I've confirmed that the truck requires at least 55-60 psi to start properly for injectors to open, and fuel pressure holds at idle (45–59 PSI), but fails to maintain when off. Based on this, I suspect a failing fuel pump or faulty check valve. I'm seeking advice on diagnosing and fixing the root cause of these lean codes and starting issues.

Related fault codes
P0171P0174
Comments (7)
Anonymous 2009-07-14 15:03

A vacuum leak is a likely culprit. Inspect all intake hoses for looseness, tears, or damage. Also check air duct clamps—ensure they are secure and not torn. Unmetered air entering the system can cause lean conditions, leading to P0171 and P0174 codes. While MAF sensor failure is possible, it's less common and usually results in more severe symptoms like rough idle or stalling. Avoid replacing sensors without confirming the root issue first.

Anonymous 2009-07-14 16:07

Thanks for the input. I've tested fuel pressure and currently reading 44 psi under normal conditions.

Anonymous 2009-07-14 19:19

Does the fuel pressure remain stable when the engine is off? If it drops immediately after turning off, this could indicate a faulty check valve in the fuel pump or a leak in the fuel system.

Anonymous 2009-07-14 20:18

No fuel pressure when the key is turned on. Pressure builds up only when the engine starts and drains completely once the engine is shut off. This strongly suggests a defective check valve at the fuel pump. While I'm not certain, this appears to be the most likely cause of both the starting issue and lean codes.

Anonymous 2009-07-15 12:39

If pressure drops so quickly and the engine starts smoothly without roughness or misfires, it's unlikely that a fuel injector or regulator is leaking. Leaks typically result in rich conditions or misfires—rather than lean codes like P0171/P0174. This points more clearly to a failing fuel pump or its internal check valve.

Anonymous 2009-07-15 13:49

The 2007 GMC 5.3L engine typically requires around 60 psi of fuel pressure before the injectors open during cranking. Since my readings are below this threshold, it's likely that a new fuel pump is needed. Additionally, replacing the fuel filter as part of the repair is recommended to ensure optimal performance and longevity.

Anonymous 2009-07-16 06:17

Fuel Pressure Readings: KOEO (Key On Engine Off): 55–62 PSI Idle: 45–59 PSI These readings support a fuel pump issue. The drop in pressure when the engine is off and the low cranking pressure align with symptoms of a failing fuel pump or defective check valve, consistent with lean codes P0171 and P0174.