Here is some aditional diagnostic information obtained this morning. After the vehicle has been running, when the ignition key is turned to the off position, the measured fuel pressure drops as follows:

Running: 50 PSI
5 seconds: 30 PSI
10 seconds: 20 PSI
15 seconds: 15 PSI
30 seconds: 10 PSI
45 seconds: 5 PSI
60 seconds: 3 PSI
90 seconds: 1 PSI
120 seconds: 0 PSI

The bottom line here is that the new pump (and the fuel gauge sender which are integrated into the replaced assembly) appears to be working as designed, but the fuel line quickly loses its pressure (it should stay at or very near 50PSI) immediately upon fuel pump shut down. Since there are no fuel leaks, the only place the fuel can be going is either into the engine cylinders via faulty fuel injectors, or back into the fuel tank. I believe that in my case if this was due to a leaking fuel injector(s), then I would have observed this prior to replacing the fuel pump. Additionally, I would think that it would take several hours (vs. seconds) to fully depressurize the fuel rail and the engine would shake/shudder upon starting due to the amount of raw gasoline that would have been leaked into the combustion chambers. What I see is that once I fully pressurize the fuel system by cycling the ignition key on and off about 10-15 times (not actually starting the vehicle, just letting the fuel pump run each time for the fraction of a second before it stops) the engine starts instantly and runs smoothly. This is why I am of the opinion that the most likely cause here is a defective backflow valve or regulator in the fuel pump assembly.

Does anybody know exactly how the fuel pump assembly is supposed to work? Am I correct in assuming that it should not allow the fuel to drain back into the tank? Thanks in advance. Where is this vacuum line going to the fuel rail? Does it attach to the tank itself since that is all that I had removed and all was working fine with the old pump.

Thanks for your thoughts.

Bob