1 High pressure pump
2 Fuel metering valve
3 Fuel rail
4 Fuel pressure sensor
5 PCM
The following comprehensive tests are carried out:
โข Measuring the differential pressure during engine start (comparison between desired and actual fuel pressure).
โข Measuring the differential pressure during engine running (comparison between desired and actual fuel pressure).
The diagnostic system classifies faults in the fuel metering system either
โข as control faults, in other words the pilot injection is switched off or
โข as function faults, i.e. the engine is switched off.
Note: control faults can also be caused by defective fuel injectors.
Switching off the engine
Because of the way the diesel engine works, the engine can only be switched off by interrupting the fuel supply. In the case of the fully electronic engine management system this is achieved by the PCM specifying “injected quantity = 0”. The corresponding fuel injection solenoid valves are no longer actuated and the engine is switched off.
Pressure drop after engine is switched off The pressure reduction is achieved by applying a current to the fuel injector solenoid valves at short intervals. Each time, the pick-up current is enough to open the control valves, but remains low enough not to lift the injector needle and thereby cause an undesired injection of fuel.
The pressure is completely dissipated within a few seconds when current is applied to the solenoid valves. After the engine is switched off, the pressure drop can be heard in the form of a buzzing noise.
Note: Before opening the high-pressure system, follow the safety precautions in the current Service Literature.