I'm looking for some opinions. Truck is a 2002 F350 and I currently have the three Dieselorings fuel pressure shim springs. As of right now, i have the black spring in because pressures using the silver and gold were less than expected. Fuel pressure with the black spring in should be between 67-70 psi but I am running a constant 59 psi. This is primarily the reason why I have the black spring in. I know one of the reasons that could be causing this is a weak fuel pump. I've never had a problem with this and the truck only has 178K miles, however, this can't be disregarded as a reason. Note that I purchased this spring kit around the same time that I installed a fuel bowl rebuild kit. The last time I researched this issue I believe it was suggested that I should bypass the fuel tank and see what my pressure is when the pump is directly sucking from a portable fuel tank. Do you think this would be a good start for investigating the issue? Right now the tank is amount 1/2 full and I really don't drive the truck much. Are there other reasons you can think of that could be causing this other than a weak fuel pump? Thanks guys

 

So I could think of a few things to consider. You could have a clogged tank pickup screen in the fuel tank or a cracked that collapses under suction. If you have a pre-pump fuel filter it could be clogged. You could be getting air in the line on the suction side. That can lower pressure.
On the return side, you could have a stuck regulator or damaged O-rings in the fuel bowl rebuild kit allowing some bypass.
Have you checked if the seat is worn where the spring presses in the regulator housing? Inside the fuel bowl, you could have a screwed up regulator poppet or pinched O-rings.
I'd also check for hairline cracks in the regulator housing. When we part these trucks out, we have seen that.
Something else to consider - A weak pump often shows up as a pressure drop under load/WOT rather than at idle. If your truck maintains 59 PSI steady, that’s slightly more indicative of a regulator leak issue rather than pump volume loss.
I think your idea of trying the external tank is a solid idea. Run the pump from a clean portable tank directly into the pump inlet. If pressure jumps up then you know the problem is on the supply side. If it’s still 59 PSI look harder at the regulator or the pump itself. You could also deadhead test it. Pinch the return line after the regulator, not for long though. If your pressure shoots up past 90 PSI then your pump is fine and the regulator circuit is likely bleeding too much. If it won’t climb, then you know the pump is crap.

If I were to give it a blind guess, because your pressure is consistently 59 PSI regardless of spring, I'd rule out the pump. The regulator assembly is prob bleeding off pressure rather than the pump not supplying volume. A weak pump usually shows dropping pressure with RPM/load, not a flatline under spec. If you need any parts to fix it, let me know. We part these trucks out. Everything is used but all OEM. Also happy to keep troubleshooting if my suggestions don't work.