A key component of the system is the fuel pressure regulator, which works with the fuel pump to maintain a steady pressure relationship between the fuel line side of the injectors and the intake manifold. It is located mounted to the fuel rail (fuel manifold).

Since the fuel is metered by the ECU by altering the amount of time the injector is held open, the volume of fuel injected would vary if there was a pressure difference between fuel supply and the intake manifold under different conditions. When there is a high vacuum in the intake manifold, such as at high rpm, the pressure in the fuel system must be reduced. When there is low pressure in the intake manifold, such as at low speed or full throttle operation, the pressure in the fuel line must be increased.

The essential requirement is that the pressure differential remains the same under all circumstances, so that only the time the injector is open determines the amount of fuel injected. In order to maintain desired fuel pressure, excess fuel is returned to the tank by a separate line from the pressure regulator.