Fuel Pressure Regulator
Fuel pressure regulator is a mechanical device located on top of throttle body on Throttle Body Injection (TBI) models or on fuel rail on 3.3L and 3.5L Multiport Fuel Injection (MFI) models. See Fig 1 . On all models, regulator is controlled by manifold vacuum. Its purpose is to maintain correct fuel pressure at fuel injector. A spring-loaded, vacuum-assisted diaphragm is located inside pressure regulator.
When fuel pump is energized, fuel flows past fuel injector into fuel pressure regulator. Regulator restricts fuel from flowing any further until proper fuel pressure is reached.
When proper fuel pressure is reached, pressure pushes on a spring behind diaphragm. As pressure moves spring and diaphragm, a return line to fuel tank is uncovered. This allows excess fuel to return to fuel tank, keeping fuel pressure constant across injector.
During acceleration, vacuum to diaphragm drops off. Without vacuum assist, spring pressure only acts upon diaphragm, resulting in a slight increase in fuel pressure before return port is opened.