Fuel system
On carburettor models, the fuel pump supplies fuel under pressure to the carburettor. A needle valve in the float chamber maintains the fuel at a constant level. A fuel return system channels excess fuel back to the fuel tank. On fuel injection models, an electric fuel pump supplies fuel under constant pressure to the fuel rail, which distributes fuel to the injectors. The electric fuel pump is located inside the fuel tank on later models, or beside the fuel tank on early models. Early models also have a transfer pump located in the fuel tank. The transfer pump acts as an aid to the larger main pump for delivering the necessary pressure. A fuel pressure regulator controls the pressure in the fuel system. The fuel system also has a fuel pulsation damper located near the fuel filter. The damper reduces the pressure pulsations caused by fuel pump operation, and the opening and closing of the injectors. The amount of fuel injected into the intake ports is precisely controlled by an lectronic Control Unit (ECU or computer). Some later 5-Series models have a fuel cooler in the return line.
Electronic control system (fuel injection system)
Besides altering the injector opening duration as described above, the electronic control unit performs a number of other tasks related to fuel and emissions control. It accomplishes these tasks by using data relayed to it by a wide array of information sensors located throughout the engine compartment, comparing this information to its stored map, and altering engine operation by controlling a number of different actuators. Since special equipment is required, most fault diagnosis and repair of the electronic control system is beyond the scope of the home mechanic. Additional information and testing procedures for the missions system components (oxygen sensor, coolant temperature sensor, EVAP system, etc.)
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