The fuel pressure regulator (Motorcraft '93-95 Lightning CM4764; '88-95 5.8L CM4760
; 5.0L CM4766) is attached to the fuel rail downstream of the fuel injectors. It adjusts fuel pressure supplied to the fuel injectors in response to manifold vacuum (engine load). The regulator is a diaphragm-operated relief valve. One side of the diaphragm senses fuel pressure and the other side is connected to the intake manifold vacuum. Fuel pressure is established by a spring preload applied to the diaphragm. Balancing one side of the diaphragm with manifold vacuum maintains a constant fuel pressure drop across the fuel injectors. Fuel pressure is high when engine vacuum is low. Excess fuel is bypassed through the fuel pressure regulator and returned through the fuel return line to the fuel tank.
A common problem is for the FPR diaphragm to fail, allowing hi-pressure fuel to leak into the red vacuum line, which dumps straight into the intake plenum. This can flood the engine, and if allowed to continue, it can wash the rings out & damage the bearings. To check it, simply disconnect the vacuum line with the engine idling & inspect for the presence OR ODOR of gasoline. It may take a few seconds for the FPR to fill with gas & begin spitting it out the FPR's engine vacuum nipple.
Some are harder to change than others due to the tiny fasteners facing down, like this '90 CV fuel rail on an '88 F150 5.0L in a '75 Bronco: