Fuel Pressure Bleed Down

1994 XJ, stock, AX-15 5-speed, 4.0 liter, 180,000 miles, 1999 intake manifold.

Reliable daily driver in stock form with the exception of the intake manifold.

Has always been a little slow to start since I have owned it for the past couple of years, but has gotten worse as of late.

So, decided to swaps the crankshaft position sensor as it still had the factory CPS. No real change in starting.

Swapped spark plugs, wires, cap and rotor since they were due to be swapped anyway. No real change in how long it takes to fire up.

Decided to check the fuel pressure. Installed a new fuel pump after I bought the XJ a few years ago. Pressure is a solid 30 psi at idle and jumps up when revved, getting up to about 40 psi.

However, when I turn the ignition and the pump primes, the fuel pressure primes to about 30psi but immediately drops to 0psi when the pump stops cycling.

So my question is, how do I determine if I have a faulty fuel pressure regulator or a leaky fuel injector?

When I swapped the plugs they looked great, not sooty, no evidence of excess fuel etc. The fuel pressure regulator is about 9 months old, the original leaked. It is an Autozone unit.

I would like some advice on how to isolate this issue.

Thanks,

Chris

Ok--DANGER WARNING--do the following:

Hook up your pressure gauge and start the engine. Have a second pair of hands turn off the engine a moment after you squeeze the return line off the regulator closed with some smooth jawed pliers. Watch the pressure. If it still bleeds off you need a check valve in the supply side. I put one on my 90 between the tank and the filter.

Got it off ebay from seller jbass195, 3/8" check valve for gas bio diesel, $18 including the shipping.

The danger warning is that if you crimp off the return line while the engine is running for more than just a moment the pressure can build dangerously high.

NOTE: LATER MODEL XJ WITHOUT THE FUEL PRESSURE REGULATOR ON THE RAIL SHOULD NOT HAVE AN EXTERNAL CHECK VALVE ADDED--REPORTED TO CAUSE BAD PROBLEMS.