fuel pressure?

[jarfly]

What is the spec on psi rateing on a 96 4.0? thank you

 

[RonD]

Should be 35-42psi at the rail

Ford used this pressure in their Return Line fuel systems, in the late 1990's Ford switched to a Returnless fuel system, it should run at 65psi.

They didn't switch all Ford models to Returnless in the same year.

If in doubt about what you have look at the fuel line(s) and look for a fuel pressure regulator(FPR) on the fuel rail.
FPR will have the Return Line attached and a vacuum hose, so a 35psi system
If there is only 1 fuel line in the engine bay then a 65psi system.

Fuel injected fuel systems should hold their pressure with engine and key off for several weeks, it is not uncommon to see a 5 to 10 psi drop with key off, but it should hold that pressure for a very long time.

 

[jarfly]

Im only getting 20lb running, around 15lb when key on.

 

[RonD]

The FPR on the fuel rail could be the problem, disconnect it's vacuum hose and plug it(so no vacuum leak), see if pressure comes up a bit.

You can also clamp the rubber Return line, being careful not to damage it, and see if pressure goes up to 35psi, if so then FPR is opening too much, bad spring or leaking seal.

Fuel pump being on should give plenty of pressure but hose from fuel pump to the top of the tank can leak, so you lose alot of that pressure and don't smell a leak because it is in the tank.

Fuel pumps tend to not come on at all when they fail, but I suppose they could start to run at a lower speed with lower pressure.
You could also check the voltage at the inertia switch in the cab, make sure it shows full 12volts when key is first turned on, if it is lower you could have corroded connection at fuel pump relay or fuse.