General information about the power supply system (BMW 5 Series E28)

All engines except early models (518) are equipped with an electronic fuel injection system.

Early 518 models are fitted with Solex carburettors. Either the Solex 2B4 carburettor is used (early models), or 2BE (late models). The mechanical fuel pump is driven by an eccentric cam on the camshaft.

Fuel-injected models are equipped with either the L-Jetronic or Motronic fuel injection system. Since 1988, fuel-injected models have been equipped with the Motronic system, which is still in use today - this system can be easily distinguished from the earlier one by the absence of a cold-start injector. The electric fuel pump is located under the rear of the car or inside the fuel tank. The fuel pump relay on the Motronic system is activated by a low signal from the Motronic control unit (ECV). The fuel pump is activated for a few seconds after the ignition is turned on and then only operates when the engine is running.

Air intake system

The air intake system consists of an air filter housing, an air flow meter and a throttle body (models with injection) and the intake manifold. This Chapter covers all components except the intake manifold; for information on removing and installing the intake manifold, refer to Part Repair work carried out without removing the engine from the vehicle.

The throttle valve inside the throttle body is actuated by the accelerator cable. When the accelerator pedal is pressed, the throttle valve opens and the air flow in the system increases.

In fuel injection systems, a flap inside the air metering unit opens wider and increases air flow. A throttle position sensor mounted on the flap shaft measures the flap angle (how much does it open) and converts this value into voltage, which is transmitted to the computer.

Fuel system

On carburetor models, the fuel pump supplies fuel under pressure to the carburetor.

A needle valve in the float chamber maintains a constant fuel level. Excess fuel is returned to the fuel tank through the fuel system return channels.

On fuel-injected models, an electric fuel pump delivers fuel to the fuel line at a constant pressure. The electric fuel pump is located inside the fuel tank on later models or near the tank on early models. Early models also have a booster pump located inside the fuel tank. The booster pump acts as an auxiliary pump to the larger main pump in creating the required pressure. The fuel system pressure is controlled by a fuel pressure regulator. The fuel system also has a pulsation damper located near the fuel filter. The damper reduces pressure pulsations caused by the operation of the fuel pump and the opening and closing of the injector. The amount of fuel injected into the intake ports is precisely determined by the ECU (ECV or computer). Some late model 5 Series have a fuel cooler in the return line.

Electronic control system (fuel injection system)

In addition to the injector opening and closing functions described above, the ECU performs a number of other tasks related to the control of the fuel and exhaust systems. The ECU performs these tasks by using data from a large number of sensors located in the engine compartment, comparing this data with information in memory, and changing the engine operating mode by controlling various devices. Most of the work involved in diagnosing faults and repairing the ECU is beyond the capabilities of the amateur mechanic due to the need for specialized equipment. Additional information and procedures for testing exhaust system components (oxygen sensor, coolant temperature sensor, EVAP system and others) are given in Chapter Engine management and emission control systems.

The article text was copied from an online resource «BMWman.ru»