Engine seems to be starving for fuel...
 

I have an old dual fuel system with an auxiliary 40L LRA belly tank, began noticing issues under load, my Xj is a 96 model and from what I understand they are a bit of a mutt when it comes to fuel system fitment.

I don't have a fuel pressure tester but have found...

Fuel rail doesn't hold pressure (or pressurise when engine running)
Fuel tank does pressurise
There are no fuel return lines

I suspect I have an external fuel pressure regulator because the hoses from the pump run to/from a device mounted nearby and the sole fuel line runs from that (see photo).

Has anyone seen one of these hybrid external FPRs before?

The issue is either a pump or FPR (or perhaps both), any thoughts pointers?

I presume you dont have the original fuel tank ?

On the '96, the fuel pump/regulator/fuel sender are one unit, in the tank...which is unique to that year

imo, you really dont have any alternative but to buy a fuel pressure tester, or have a mechanic look at it

Unfortunately the original tank is long gone, I believe the LPG conversion was done over 12+ years ago, according to the aftermarket supplier of the belly tank, the original pump/sender assembly was re-used.

Given no FPR is present on this earlier assembly variant (see picture) an external FPR was required but I cannot find anything about them online as yet.

I am reluctant to go full "parts cannon" on this but given the age of the vehicle it should be considered that the pump, sender and FPR could be approaching their use-by date.

Sadly the tank is full so I will "Grandma" the XJ around for the next 2 weeks and consider my options, looking at price vs. reward as the LPG line has ruptured and the tank will require re-certification (required after 10 years) before it can be serviced.

Vehicle has fully rebuilt AW4.

A pump/FPR/sender assembly upgrade would seem to be the easiest as apart from the fuel line the only other component is the injector fuel rail.

As Tony said the 96 pump has the pressure regulator in it I also have a 96 dual fuel if yours is like mine the fuel tank is just in front of the rear axle? if so it is easy to change the pump take the rear pass wheel off and the pump loads in from the side no need to remove the tank.
I would suggest getting a fuel pressure test kit they are only about $25 on eBay chances are the regulator is faulty and not holding pressure Now you can buy the regulator separately pull the pump out and replace it or I would just get a new pump module that way everything is new I recently got a pump module off rockauto for about $150 delivered for my 99
I would also recommend pulling the tank out and cleaning it and clean the fuel rail while you have it in bits put some commodore injectors in it that's next for my 96

there seems to be a conflict in your answer, as you say "the original pump/sender was re-used", but the '96 has an integral in-tank sender/regulator

You seem to be suggesting there is an external regulator as well ?

The '96 unit is unique to that year

99 is the same Tony sender/pump/regulator module
there is no external regulator unlike other earlier models where the regulator is on the fuel rail

All compliance (and LPG fitment) plates state 1996, however I have since found a small obscured tag on the upper drivers side firewall simply stating "built Nov 95".

The fuel pump sender combo is the side mounting type not the type installed from the top of the tank and features 2 outlets which run to what I suspect to be the FPR, the integrated FPR/fuelpump/sender combo has only 1 outlet.

My installation doesn't appear to have a fuel return line.

Fuel pressure confirmed at 45-50psi at idle and higher RPM (FP tester arrived today), pressure drops off quickly when engine shut off, pressure drops quickly even when fuel line clamped (2 pairs of pliers) attempting to hold pressure in fuel rail.