Early 518 models are equipped with Solex carburetors. Either Solex 2B4 carburetor is used (early models), or 2BE (later models). The mechanical fuel pump is driven by an eccentric camshaft cam.
Fuel injected models are equipped with either L-Jetronic or Motronic fuel injection systems. Since 1988, fuel-injected models have been equipped with the Motronic system used to this day - this system is 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 vehicle or inside the fuel tank. The fuel pump relay on the Motronic system is switched on by a low signal level from the Motronic control unit (ECV). After switching on the ignition, the fuel pump turns on for a few seconds and then only works 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 (injection models) and inlet pipeline. In this Chapter all components are considered, except for the inlet pipeline; for information on removing and installing the intake manifold, refer to Part Repair work carried out without removing the engine from the car.
The throttle valve inside the throttle body is actuated by the accelerator cable. When you press the accelerator pedal, the throttle valve opens and the air flow in the system increases.
In fuel injection systems, the flap inside the air meter opens wider and the air flow increases. A throttle position sensor mounted on the damper shaft measures the angle of the damper (how does it open) and converts this value into voltage, which is transmitted to the computer.
Fuel system
On carburetor models, the fuel pump delivers pressurized fuel to the carburetor.
A needle valve in the float chamber maintains a constant fuel level. The excess fuel flows back to the fuel tank through the return channels of the fuel system.
On models with fuel injection, an electric fuel pump delivers fuel to the line at a constant pressure. The electric fuel pump on late models is located inside the fuel tank or next to the tank on early models. Early models are also equipped with an intermediate pump located inside the fuel tank. The intermediate pump acts as an auxiliary to the more powerful main pump when the required pressure is created. The pressure in the fuel system is controlled by the fuel pressure regulator. The fuel system also has a pressure pulsation damper located next to 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 electronic control unit (ECV or computer). Some late 5 Series models have a fuel cooler in the return line.
Electronic control system (fuel injection system)
In addition to opening the injectors in turn, as described above, the ECM performs a number of other tasks related to the control of the fuel and exhaust systems. The unit performs these tasks using data that comes from a large number of information sensors located in the engine compartment, comparing this data with information in memory and changing the engine operating mode, controlling a variety of devices. Most of the troubleshooting and repair of the electronic control system due to the need for special equipment is beyond the capabilities of an amateur mechanic. Additional Information and Procedures for Testing Release System Components (oxygen concentration sensor, coolant temperature sensor, EVAP system and others) given in Chapter Engine management and emission control systems.