General information about fuel injection (BMW 5 Series E28)

The fuel injection system consists of three main subsystems: the fuel system, the air intake system and the electronic control system.

Fuel system

An electric fuel pump located in or near the fuel tank delivers pressurized fuel to a fuel rail, which distributes the fuel evenly between all the injectors. From the fuel rail, the fuel is injected into the injector inlet, located below the intake valve. The amount of fuel injected by the injector is precisely determined by the Electronic Control Unit (ECU). When the engine is started, an additional injector, known as a cold start injector (only on L-Jetronic and early Motronic systems), ensures the supply of additional fuel to the intake manifold.

The pressure regulator controls the pressure in the system in accordance with the magnitude of the vacuum in the intake manifold.

The fuel filter between the fuel pump and the fuel line filters the fuel to protect system components.

Air intake system

The air intake system consists of an air filter housing, an air flow meter, a throttle body, an intake manifold and associated ducts. The air flow meter provides information to the ECU. These models are equipped with a vane-type flow meter. The intake air flow is measured by a potentiometer, and a temperature sensor measures the temperature of the intake air. This information helps the ECU determine the amount of fuel injected by the injectors (injection duration). The throttle valve inside the throttle body is controlled by the driver. When the throttle valve opens, the amount of air passing through the system increases, the potentiometer signal increases and the ECU controls the injectors to increase the amount of fuel delivered to the intake devices.

Electronic control system

The processor-based control system operates the fuel and other systems through the Electronic Control Unit (ECU). The ECU receives signals from a variety of information sensors that monitor such variables as intake air volume, intake air temperature, coolant temperature, engine speed, acceleration/deceleration, and exhaust oxygen content. These signals help the ECU determine the injection duration needed to achieve the optimum air/fuel ratio. These sensors and their associated ECU-controlled actuators are located throughout the engine compartment. For further information regarding the ECU and its interaction with the engine electrical systems and ignition system, refer to Chapters Ignition system and Engine management and emission control systems.

Either the L-Jetronic or Motronic system is used. Later models are equipped with a modern version of the original Motronic system.