My Pathfinder has been sitting for awhile while I rebuilt the motor and now I'm running into problems from it sitting for so long.
I put a pressure gage on the fuel rail and when I turned the key on, the pump would run for about 6 seconds and turn off. The fuel pressure would go to just over 40 and when the pump shut off the pressure would drop to zero. I'm pretty sure the pressure should hold at 43-44 psi so, I'm looking for a check valve somewhere in the system on the outlet of the pump.
I have been reading on this site and others that there should be a check valve on the outlet of the pump but, other people are equally certain that there's a check valve built into the filter housing. I replaced the fuel pump with a new Bosch unit and Bosch is a pretty decent brand but, the fuel pressure still drops off. I have a new filter that I haven't installed yet but, I'm able to blow air through it both ways so, there doesn't seem to be a check there. What am I missing? Do I have a defective pump? Lately I've been running into more and more new parts with defects. I changed out all the injectors only to find that one of them was stuck wide open causing a bad spark knock and lots and lots of white smoke. Any insight would be greatly appreciated.

You should check the fuel pressure regulator, that's the "return" to the tank and will cause a drop like that if it's leaking or stuck open. When it's operating normally the pressure will hold in a "no vacuum applied" state for awhile after priming. Check for fuel in the regulator vacuum line (leaky diaphragm) and try pinching off the return line to the tank. If it holds pressure with the line pinched then your problem is on the return side and not the pressure side.

Thanks for your reply. I already replaced the pressure regulator once but, I just bought another one to replace it again. The quality control of new after-market parts is frustrating.

Throwing parts at it is just going to cost you bucks and get you frustrated, you need to isolate where the pressure is going. Pinch off the return line, that's a conclusive test. If that makes it hold pressure then the regulator is bad, if it doesn't then the regulator is fine and the problem is someplace else. You can do the same thing to validate the rail and injectors, pinch off both inlet and outlet after priming and see if the pressure holds. Same for the pump itself, put the gauge at the pump outlet and then pinch the line at the fuel filter. With some simple tests you can at least narrow the list of suspects, if not isolate the culprit.

Thanks for your help Vstar. I only bought the pressure gage kit recently so, I'll be taking a more methodical approach from here on out.

Vstar, if I could ask a follow-up question.
I did a little playing around this morning with the pressure gage kit. I pinched the hose before the rail inlet and the pressure held as designed so, the check in the pump works, it held at over 50#. Next I pinched the line at the rail outlet and the pressure dropped off as before so, I have eliminated the fuel pressure regulator and determined that I once again have an injector stuck open. Going forward I will lift the rail and injectors to determine which one it is. The last time it was #4 cylinder and if it is #4 again I think I may have an electrical problem rather than an injector stuck open mechanically. I have a hard time believing that brand new injectors have such a high failure rate so, I'm wondering if the signal to the injectors comes directly from the ECU or if there is an intermediate module causing an injector to stay open electrically. I was able to locate a used ECU from a wrecked car and it seems to make no difference. As far as throwing parts at the car, well, it's 25 years old and I plan to drive it until EVs are mandated. I just rebuilt the motor down to the crankshaft journals and recently had the car painted.

For an injector to cause pressure loss with the key off, one of two things is happening, either one of the o-rings is leaking or the injector plunger is stuck physically open. The supply voltage to the injectors is cut off when you kill the ignition, and the ECM is dead so there's no potential ground either (there's no intermediate module, the ECM drives the ground side of the injectors directly). That means an electrical problem would require a simultaneous failure on both sides of the circuit, which is extremely unlikely.

Did you put in new injectors, or remans? The failure rate on remans is hideous, even from reputable manufacturers. I've seen and heard of many instances where 50% or more of a batch were malfunctioning or DOA. The failure rate on China-cheap new ones isn't zero either, although it's better than with remans.

That was quick. It's great to have a Pathfinder specialist on stand-by.
The pressure drops off with the key in the ON position. I will go out and try turning off the key before the pressure drops and see what happens.
The injectors were sold as "new" on Ebay for about a hundred dollars for the set. I guess there's the possibility that the harness wire to the injector is grounding out somewhere.

Pressure drops off with the key on or off so, I guess I'm dealing with a bad injector. At least I've eliminated an electrical problem. Better a mechanical problem than electrical.
Thanks again for your insight.

You're most welcome. You can probably isolate which injector(s) are bad just by pulling the plugs and sticking a clean q-tip down each hole after priming the pump and letting the pressure leak down. The human nose is exquisitely sensitive to the smell of raw gas, so the stink will generally betray the leaker.

Unbelievable, three of the injectors were free flowing. 50% fail rate. Rebuilt Bosch injectors are 200-250 for a set of 6 but I want to stay away from rebuilt. New Bosch units are more than twice as much. There must be a safe middle ground. Options are limited here in Costa Rica. Maybe I'll just throw Chinese injectors at the problem until I get 6 that work and pick up a set of OEMs when I get back to Florida.