In multi-point injection systems, the fuel pressure regulator is located on the outlet from the fuel rail.
Fuel circulates continuously, but the fuel pressure regulator controls the quantity of fuel returning to the tank. This in turn controls the pressure in the fuel rail.
It consists of a metal housing with a spring-loaded diaphragm between the two halves of a steel pressed shell. A valve attached to the diaphragm controls the exposure of the opening to the return line.
With the pump operating, fuel pressure builds up on the underside of the diaphragm, and raises it against the force of the spring. The central valve is carried with it, which lets fuel return to the tank. Fuel in the rail is now kept under pressure that’s determined by the force in the spring.
As well, further control is needed, as fuel pressure has to stay at a constant value above manifold pressure. Otherwise, the fuel quantity delivered won’t be accurate.
The spring housing above the diaphragm is sealed, and connected to the intake manifold. Low pressure in the manifold then acts on the diaphragm and helps its movement against the spring. As a result, fuel pressure is continually corrected in step with changes in manifold pressure.
When manifold pressure is low, as at idle, fuel pressure will be lowered. When manifold pressure is high, as at full throttle, fuel pressure rises. As a result, the quantity of fuel injected into the engine is set solely by the duration of the electrical pulse from the control unit.