The engine management system continually compares the fuel pressure request (calculated by the system) and the actual fuel pressure in the fuel rail (measured by the fuel pressure sensor).
If the system is working properly, the two parameters will be within ยฑ 50 bar of each other.
If they differ by more than -50 bar (for example the required fuel pressure = 350 bar, but the actual fuel pressure = 290 bar), the fuel quantity to be injected is set to 0 and the engine cuts out or cannot be started.
The reason for this is that if the pressure difference is outside the tolerance, it is no longer possible to carry out a controlled injection process.
The cause may be faulty fuel pressure measurement or faulty fuel metering; however, it is also possible that there could be a leak at the fuel injector solenoid valve.
If there is a leak at the fuel injector solenoid valve, the fuel that has leaked is conveyed via the leaking solenoid valve into the leak-off pipe.
The result is an increased quantity of leak-off fuel, which is supplied to the fuel return line via the leak-off pipe.
This increased quantity of leak-off fuel can be measured using a special tool (a measuring container for each fuel injector) which is connected to the leak-off pipes of each individual fuel injector.
After carrying out the measurement as specified (see current Service Literature), it is possible to tell from the individual quantities of leak-off fuel, whether there is a leak at the fuel injector solenoid valve and which fuel injector it belongs to.
A difference of more than +50 bar might indicate a blocked fuel injector. A blocked fuel injector is not able to open fully as required.
This means that it is not possible to fully reduce the pressure in the fuel rail as calculated. As soon as the difference in pressure rises above +50 bar, the quantity injected is set to 0.