Mystery Fuel Pressure

[ireburb]

I have had what I consider low fuel pressure (5 psi at idle and 1psi at WOT) before the IP and can’t seem to chase down the issue. Been working at this for a while now. My fuel pressure gauge is tee’d between the FFM and the IP. Here is a complete list of what I have tried. The list is not in order:

Dropped tank and replaced sock.
OPS replaced.
Replaced fuel line from tank to lift pump.
FTB Modification.
Fuel Filter replaced.
Relay activated lift pump (OPS still shuts off relay as it should).
Replaced fuel pump relay in engine fuse box (Not sure what this does anyways but it was cheap).
Fuel line replaced between filter and IP during FTB mod.
Clear hose just before IP and no bubbles present.
Replaced lift pump with 93 model lift pump (First 93 model failed)
Installed dedicated ground for lift pump.
Ran lift pump on 12v source (jumpstart battery pack with leads plugged in to lift pump).
Installed redundant mechanical fuel pressure gauge in engine compartment (in case electric one fails).
The only thing I can think of left to do is blow out the line between the lift pump and the FFM. I was able to get 9 PSI without the truck running and running the lift pump on a dedicated battery so I know the lift pump is capable of producing 9 PSI. I have no stalling or stuttering. WVO fuel pressure is 9 PSI idle and 6 PSI WOT. I also plan to install a gauge between the diesel lift pump and the FFM at some point and take measurements but I don't expect this will yield anything.

Any other ideas to try?

 

[Matt Bachand]

What pressure does the pump 'deadhead' itself at WTS or jumping pump without truck running?

 

[ireburb]

It has been a while since I ran this test but I think it was around 9psi.

 

[buddy]

Plain and simple, any stock lift pump cant provide a power programmed truck enough fuel. You need a better lift pump. The IP is pushing 25-30% more fuel than you were before.

Also, the fitting in the IP for the return line is a pressure regulator. It may be letting a little more flow through than its supposed to. But its supposed to be 5psi, and it sounds like thats what you got at idle when there would be excess supply.

 

[bison]

IMO,5 psi at idle and 1 at WOT is pretty much standard,as long as there's some pressure the engine aint starving

 

[mfgguru]

My truck runs 5-6psi idle and about 2.5psi WOT. I have a FTB mod. It is a Carter stock LP, but for now it gives me all the fuel I need. That may change once Wester's is on board. We will see.

 

[Acesneights1]

IMO,5 psi at idle and 1 at WOT is pretty much standard,as long as there's some pressure the engine aint starving

Yep, that's what my stock 94 does and I have a mech test gauge on it right now.