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My neighbor's 2010 Navigator has a cold start issue and a P0172 rich code. This started after the fuel pump was replaced. The injectors have been replaced, the MAF sensor has been cleaned and there are no apparent vacuum leaks. The fuel rail pressure seems low at 32psi as the spec is 50-55psi but the fuel pump has just been replaced. The koeo rail pressure does not drop over 5 minutes. The bank 1 LT fuel trim stays at -25. I can see where the low fuel pressure could  cause the hard start but I wouldn't think it could result in a rich condition. I'm at a loss as to where to go from here.
Vehicle: 2010 Lincoln Navigator 5.4L

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The specification for the fuel pressure is 55-60, The pressure you have 32 that to low , If this engine has a direcrt fuel injector with a high pressure pump, with a low rail pressure the fuel would not atomize correctly the engine would run rough. Even if the engine does not have a direct fuel injector, the fuel pressure is to low, And you would be correct to say the fuel trim would go low. and the system would be lean.  You would also set a fuel rail sensor code, a performance code P0191. With 32 psi at the fuel pressure the engine would still run and  its correct it would have a hard time to start up. You already tested the fuel injector with the 5 minute hold. A low fuel pressure can create a bad spray pattern on the fuel injector it should set a P0172 with a P0300.

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