Looking on epc with serial number it shows the cool fuel 3 system that has a regulator return fuel line to the water sep housing
There also is a vacuum reference line from the intake manifold to the regulator. If the system is connected and sound should not the fuel pressure read well above spec when the vacuum line is disconnected?
I have 7.4L L29 mpi in my head due to a recent over fueling issue. That was a fun one
Having/using the correct fuel pressure regulator installed is to help match what the fuel injector flow rates are as the injectors are different number wise between engine sizes and years usually spelled out in in the descriptions like as noted above.
It started with 12 psi but got bumped up to 30 psi on the quick.
So some engines need 30 psi, some of the 454 mag mpi's where around 36 psi +/- 4ish psi, then when to other injectors types/sizes/brands and the psi followed accordingly.
Some engine models are 40 or 42 psi depending on what manual you're reading that use different injectors, and some engines are 43 psi to start and all have about 4is psi up or down with the vacuum line R&R'ed.....Just depends on what Merc EFI/TBI/MPI engine you're working with at the time.
All regulators move 4 or 5ish psi as that is the diaphragm inside using vacuum to help control the flow (or pressure) of the pump if you will... All the electric pumps are capable of producing a high number of over 60 to around 100 psi when blocked off to build pressure and actually done as part of a fuel pump test mentioned in service manuals to help tell us if the pump is working correctly/up to the task by blocking its return...kind of like why when you see the little screen under the regulator gets dirty the psi goes really high...I've read close to 100 psi at times.
But yes, the 2 items have to match to work it's best, and it appears the op may have accidentally installed the wrong regulator as his posts show he went from 28/32 to 30/38 but needs a 43 numbered one to start out with in which the next new one may show 38+ish psi with vacuum line connected and bumped up when disconnected... This mentioned, thus far the only complaint of the post seems to be low fuel pressure once again on the gauge but not how the engine is actually running/performing as he's probably just testing at home. The engine may or even may not start/idle but it ought to not run normally being a bit low on pressure for what the injectors call for if the engine starts at all.
Searay with a 5.7 MPI engine, bravo drive, serial # ow027371 that had low fuel pressure, 28psi and 32 with reg unplugged. Replaced both pumps with OEM units and the fuel regulator and filter screen down below and only getting 30PSI and 38 with regulator unplugged.
