A pressure regulator is a part of the car fuel system that maintains a constant pressure of the fuel and returns any excess fuel to the fuel tank. Using a diaphragm linked to the engine vacuum, the regulator maintains the pressure at a preset level.
The amount of increase or decrease of fuel pressure controlled by the regulator depends on the engine load and speed. Fuel pressure must be higher when the load is higher and vice versa. Almost all modern cars have “returnless” electronic fuel injection systems, meaning that the regulator is inside the fuel tank with no need of a fuel return line. Fuel pressure is generated by an electric fuel pump that harbors the regulator in the case of returnless electronic fuel injection systems.