The figure depicts the two-module system; with the single-module system the IDM is integrated in the PCM.
A Fuel injection line
B High pressure line
C Fuel return from pump to tank/filter
D Fuel feed
E Leak-off pipe
F Fuel return to tank
1 Fuel injector
2 Fuel rail (common rail)
3 High pressure pump
4 Fuel filter
5 Fuel tank
6 IDM *
7 EEC V-PCM *
* On newer systems these are combined into one control unit
Function
The fuel is drawn from the fuel tank via the fuel filter by means of the transfer pump integrated in the high pressure pump.
The high-pressure pump compresses the fuel and forces it into the fuel rail.
The fuel pressure required for any given situation is available for the fuel injectors for each injection process.
Leak-off oil from the fuel injectors and/or returning fuel from the high pressure pump are fed back into the fuel tank.
Possible causes of defects in fuel pipes and fuel tank
Fuel lines may be blocked due to foreign bodies or bending.
In addition, blocked parts and lines of the low-pressure system can cause air to enter the low-pressure system on account of the increased vacuum in the system.
Air can also enter the low pressure system through loose or leaking pipe connections.
Faulty valves or pipes in the tank venting system can impair the flow of fuel through the low-pressure system.
Effects in case of faults (low pressure system contains air or is blocked)
Poor engine starting when warm or cold
Irregular idling
Engine does not start.
Engine starts, but cuts out again immediately afterwards.
Engine has insufficient power.
Note: At a certain residual fuel amount, the PCM causes the engine to judder. The intention is to draw the driver’s attention to the fact that the vehicle must urgently be refueled.
Note for vehicles with EOBD: If the system causes the engine to judder because the fuel tank is empty, the EOBD is deactivated during this phase. This prevents apparent faults from being displayed.