A high-pressure electric pump draws fuel from the fuel tank and delivers it to the metering unit. A pressure relief valve is installed between the fuel pump and the metering unit to regulate fuel line pressure by bleeding off excess fuel back to the tank.

The metering unit is a pump that is driven by the engine camshaft. It is always in the same rotational relationship with the camshaft, so it can be timed to feed the fuel to the injectors just at the right moment.

Each injector contains a spring-loaded valve that is opened by fuel pressure, injecting fuel into the intake at a point just before the intake valve.

The throttle valve regulates engine speed and power output by regulating manifold vacuum, which, in turn, regulates the amount of fuel supplied to the injectors by the metering pump.

The more common type of timed fuel injection is the electronic-timed fuel injection, also known as electronic fuel injection (EFI) (fig. 4-44). Anelectronic fuel injection system can be divided into four subsystems:

1. Fuel delivery system

2. Air induction system

3. Sensor system

4. Computer control system

The fuel delivery system of an EFI system includes an electric fuel pump, a fuel filter, a pressure regulator, the injector valves, and the connecting lines and hoses.

Figure 4-44. - Electronic-timed injection.