Fuel Pressure Regulator
WARNING: This page does not describe the selected car, but rather 10 other vehicles, including the 2002 Mazda Tribute, 2002 Mazda Protege5, 2002 Mazda Protege, 2002 Mazda Millenia, and 2002 Mazda MX-5 Miata. However, it is still accessible from the selected car via links, so may be relevant.
The fuel pressure regulator is a sealed unit, divided into 2 chambers (fuel and spring) by a diaphragm. The fuel chamber receives fuel through the inlet side of the fuel injector rail. The spring chamber is connected to intake manifold vacuum.
At idle, intake manifold vacuum is high. The diaphragm is pulled back by intake manifold vacuum, and excess fuel is returned to the fuel tank. As throttle opens, intake manifold vacuum decreases. The regulator spring overcomes manifold vacuum, increasing fuel pressure.