Fuel injection systems (BMW 5 Series E28)

L-Jetronic fuel injection system

The Bosch L-Jetronic fuel injection system was used on most E28 ("old style") models. This electronically controlled fuel injection system uses one solenoid injector per cylinder. The system is controlled by an Electronic Control Unit (ECU), which processes information from various sensors and then delivers precise fuel dosage to the cylinders by regulating the injector open time. An electric fuel pump delivers high-pressure fuel to the injectors via fuel lines and an in-line filter. A pressure regulator delivers fuel at the optimum pressure, allowing the pressure to drop and rise depending on engine speed and load. Any excess fuel is returned to the fuel tank via a separate line.

A sensor in the air intake duct continuously measures the amount of incoming air, and the ECU adjusts the fuel mixture to achieve the optimum air/fuel ratio.

Other components that make up the system include the throttle valve (which controls the air flow), coolant temperature sensor, throttle position sensor, idle speed stabilization valve (which, to stabilize idle speed, removes air, bypassing the throttle valve) and associated relays and fuses.

Motronic fuel injection system

The Motronic system combines the fuel management of the L-Jetronic injection system with the control of ignition timing, idle speed and exhaust in a single unit.

The fuel injection and idle speed control functions are the same as those described above for the L-Jetronic system. For more information on the Motronic system, refer to Chapter Engine management and emission control systems.

The oxygen sensor on later models with a catalytic converter is installed in the exhaust system. This sensor continuously measures the oxygen concentration in the exhaust gases. This information is used by the ECU to adjust the injection duration, allowing the fuel mixture to be set for optimum converter efficiency and minimum emissions.