My 01 has a continuing issue that has me pulling my hair out.
Background
It has been doing this for a long time as in well over a year.
I put in a cam and at that time pulled my rasp fuel pump and installed a mechanical fuel pump off a 12 valve. It did this with the rasp pump and does it now so the fuel pump is not the issue(and I have tried two different 12 valve pumps). I have a bypass installed to deal with the excess pressure. I put in a new bypass (the rasp pump had one too). It does it with or without the bypass installed so it is not the bypass. The bypass fuel flows back to the tank into a vulcan performance modified liberty module that has an extra return port along with a drawstraw. I replaced the module as my stock one had a tear in the screen and I thought it might be sucking that torn screen flap blocking the suction line but it still does it. Long ago I replaced the line from fuel filter canister to the VP44. I have replaced the lines from the tank to the hardline along the frame rail. The only stock lines left are the hardline along framerail and the stock return line. I am still using the stock filter setup. There are no apparent leaks on either the suction side or the return side. I have plumbed in a temp second gauge to verify the accuracy of the original gauge. One is prefilter the other post filter. They both mirror each other pretty close. My VP went out long ago and it did it with the old one and does it with the new one. I have replaced the check valve on the return line of the VP44. Fuel cap on or off (to test for a vacumn in the tank or bad vent) makes no difference. Air temp and fuel temp make no difference. Load makes no difference. Truck warm or cold makes no difference. Short drive or long all day drive makes no difference. Truck runs fine when it does this it has never bucked even at 30psi boost pulling a trailer up a mountain pass. Starts fine everytime.

Now for what it does.
It will randomly lose pressure. I have it set to run at 16-17psi and it might drop to 12, or 10 or 2psi. Generally if driving down the road and it does it I will slide into neutral and rev the engine a couple times and the pressure will come back. Might lose pressure later that day or in two weeks. The 12 valve mechanical fuel pump can/will produce 39psi on my edge j/a monitor and will peg my test gauge past 30psi without the bypass installed. I am just under 400hp so both the rasp and now the 12 valve mechanical pump are more than capable of pumping enough fuel for it.

I am stumped and looking for ideas, even wild ones.

 

The first thing I would suspect with your setup is the pressure gauge sending unit. You say you installed a redundant gauge - if a mechanical gauge duplicates the problem then I am at a loss. If it is also an electric one, I would check the power supply to both of them to see if it drops the voltage once in a while.

 

The gauges- one is electrical the other is mechanical. Your suggestion was one of the first I checked which is why I plumbed in the mechanical gauge temporarily is it still temporary after a year?

 

There is nothing more permanent than something temporary. My gauges are still cable tied to the steering column, 1.5 years after I installed them and said, hey, this will do for a week or two while I make brackets...
If the pump is delivering pressure (which a mechanical pump should), the pressure relief bypass is the only other part that has to do with pressure. It it gets stuck open for some reason that would do it (although for a spring loaded mechanical bypass that is hard to imagine).

 

With the bypass I have installed for the pump it does it with or without it in place. Now as far as the bypass in the VP44 don't know about that. Though it has done it with more than one VP.

While following this line of thinking if it isn't a pressure relief issue then it must be some sort of intermittent blockage in the supply line. which is hard to imagine but.... I have been putting this off but I guess I should replace the line from the tank to the pump and bypass the hardline along the frame rail as well and see what happens.

 

Bizarre. Hard to imagine a line would do that. Is there any kind of a cross-flow or bypass valve in the pump itself? I am not familiar with the design but I am curious now, I may look in my FSM.

 

My lift pump is just your standard cummins mechanical lift pump out of a 98 12 valve. Nothing special. It does not appear to have anything other than inlet and outlet. However it did it before the cam and pump swap when I was using a RASP mechanical belt driven fuel pump as well.

 

what's the fuel level when the pressure drops?
less than 1/4 tank..
I had mine loose pressure with an AD100 at times when its below 1/4 tank.
I think it sucking too much air.

-dkenny

 

Fuel level makes no difference. I have installed a Jeep liberty modified fuel module last year (it has a drawstraw preinstalled in it).
http://www.vulcanperformance.com/Dra...w-p/jlfmds.htm
My stock canister had a torn screen in the bottom and was thinking it might be sucking up that flap of screen causing a restriction but that was not it.