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I just changed out the fuel pump and very dirty fuel filter on my 2007 4.2 SC, and I still do not have the required 45 psi (4.5 bar). When first starting, fuel pressure goes up to 59 psi, then within 10 seconds will drop down to 30-35 psi. The fuel pressure does rise past 45 psi while driving, and I have noticed a substantial increase in power since changing the pump.

I do have a P0174 code which seems believable, considering the low fuel pressure at idle. I am wondering if my fuel pump controller is somehow faulty? As usual, any help appreciated.
 

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This system uses a variable fuel pressure/volume output. You will need to compare the fuel pressure gauge reading to the actual and target fuel pressure readings on a scan tool. The ECM sends a command to the fuel pump driver module to increase or decrease fuel pump output depending on fuel pressure demand. Check the fuel trims for both banks. I would suspect that a fuel pressure issue would affect both banks and set both P0171 and P0174 codes.
 
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