Iím testing the limits of my LB7 knowledge and encourage old timers to weigh in...please.
To be clear, the engine you now have in the truck has its original
fuel system (CP3, lines, injectors, etc) and that engine sat since 2005.
To answer your question about failed return circuits in the injectors, the CP3 provides a high pressure, low volume supply of fuel and even one sketchy ball/seat in one injector would act like a hydraulic short circuit, and rail pressure would not rise to a level necessary for starting. A more likely scenario would be all the injectors may be somewhat comprised, with the same effect on rail pressure.
67,000 km is about 47,000 miles which would put your replacement engine well below the 70k +/- time to failure of typical LB7 injectors, but all the time your replacement engine was on the shelf is worrisome
. As you know, diesel fuel has a limited storage life and, just speculating here, but I wonder what evil old fuel could have been doing to the system over all those years?
You mention you can hear fuel returning to the tank.
The return test I suggested only amounts to a quite small amount (? Donít recall the number but mmís) during a 15second crank and it seems unlikely to me there would be enough flow to hear. Where do you hear it?
Seems to me a good test of the CP3 would be to find a gauge of appropriate pressure range and connect it to the CP3 output. Measuring pressure there would seem to eliminate downstream problems. The fact that when you put fuel directly to the CP3 and it immediately came out the return side may be what the police call ďa clueĒ.
Iíll stop rambling now, and wish you the best.
Hereís an LB7 fuel system diagram and an internal view of an injector, showing the return circuit, that may be helpful: