Very Low Fuel Rail Presssure
 

A while back I posted a similar thread for the same problem it has since gotten worse so I'm posting a new thread in hopes of some help with this issue. I have a 2005 F150 w/ 5.4. I am getting a very low frp (fuel rail pressure) randomly on start-ups. It's gotten more frequent as of late. My gryphon programmer is showing me a low psi range a couple of times a day with the key in the on position but not crank. By low I mean anything below the normsl 39-45 psi. It's read .02-25 psi. When under 6 psi and cranked it will not start or is subject to long cranks. Over 6 psi I can usually get a start. When the frp kicks in low there is a slight hum and then the hum stops instead of the high pitch I usually hear when the fuel pump kicks in. The truck drives fine and the frp never falls below 38 psi once the truck is started. I've been to the dealer and they can't get repeat nor find any code. I also cannot retrieve a code with the gryphon. I've just recently changed the fuel pump driver module in the hopes it was bad. It was not and the problem persists. If I have to have the fuel pump changed so be it but I would like to know if maybe the fuel pressure sensor could be the issue. Or if their is a fuel pump relay I maybe able to change or anything else I may be missing. Please help any experience or advice is greatly appreciated.😀

I should add that I've changed the fuel filter also and used STP fuel system cleaner to try and clean any injectors that may have been dirty or clogged. Please if anyone has any ideas I'm getting desperate. I really don't want to spend money on a new fuel pump just to have the same problem keep happening.

Can you monitor FPDC (Fuel Pump Duty Cycle) with your gryphon programmer? (PID # 1672) If it is flaky corresponding to your low Fuel Rail Pressure, it might point at the Fuel Pump Driver Module, although replaced, or a poor/flaky electrical connection in that direction.


There is also a Fuel Pump Output Fault 'flag' available. Might require ability to input custom PID and formula - it is at PID # 162E, bit 2 (Hex 04).


I do not know if (or when) either of these will set a DTC.