The FPR on 1997 and earlier Rangers has the Return Line attached to it, and a vacuum line, the vacuum line is what regulates engine Load fuel pressure
The IN fuel line from the filter attaches directly to the Fuel rail, on a 4.0l OHV that's the part between upper and lower intakes
The stock FPR's internal spring is set for 42-44PSI
This is what pressure should show with no vacuum hose attached
With hose attach and engine at idle, vacuum is HIGH so that pulls FPR open a bit more reducing fuel pressure to 35psi, approx.
When you accelerate fuel demand goes UP and Vacuum goes DOWN, this helps stabilize pressure at 35psi
Same for engine Load, which is directly related to Vacuum in the intake, light load high vacuum, high load low vacuum
So pretty simple system
Without this Vacuum adjustment the computer has a hard time calculating OPEN TIME for the injectors, so you can get the stumbling when going from low load to high load
Say pressure is 43psi and low load(no vacuum hose attached)
When you accelerate, fuel flow increases and pressure drops to 30psi, or less, so computer's calculated 43psi injector open time is WRONG, so it has to increase open time for lower pressure, and it does, but YOU feel that as a stumble, no instant throttle response because fuel pressure is not Stable
Adjustable pressure is what there is with stock system, in working condition, it adjust to engine load which is what you want
Higher fuel pressure doesn't give any engine more power, fuel injected or carb
Gasoline engines need 14.7:1 air:fuel mix
Adding more fuel gets you a flooded engine, not more power
TPS(throttle position sensor) this is used by the computer to give instant throttle response
Its a 5volt sensor, like most
Works like a light dimmer or volume control, variable voltage/resistor
Computer "sees"
0.9volt closed throttle
4.5volt wide open throttle
If there is a dead spot, at say 1/2 throttle where voltage drops suddenly then there would be a hesitation as computer "thinks" you took your foot off the gas pedal and turns OFF the fuel injectors
Very very rare failure
One of the fuel saving benefits of fuel injection is that the computer can turn OFF all the fuel injectors when your foot is off the gas pedal when coasting, carbs just kept on sucking in fuel, lol
Computer turns injectors back on when RPMs are under 1,500 or so
It does intermittently turn on injectors to keep Cats hot when coasting