Releasing Pressure In High Pressure Area

Reducing Pressure

Special tools, testers and auxiliary items required

• Vehicle Diagnostic Tester


There is a risk of injury because the fuel is under very high pressure.

• The fuel injection system is separated into a high-pressure section (maximum approximately 120 bar) and a low-pressure section (approximately 7 bar).

• Before opening high pressure parts, for example removing high pressure pump, fuel rail, fuel injectors, fuel pipes or fuel pressure sensor (G247) , fuel pressure in high pressure area must be reduced to the recommended pressure. The procedure for this is as follows.

Reducing Fuel High Pressure:

- Connect a vehicle diagnostic tester.

- Start engine and run at idle speed.

- Select "engine electronics" in OBD.

- Select "Measured values".

- Select "Fuel pressure" from the list.

- When the engine is at idle, fuel pressure between 30 and 40 bar is displayed.

- Pry the cover - 1 - on the left and right of the instrument panel off, for example using the trim removal wedge (3409) - arrow -.






- While the engine is idling, remove the fuse - in the fuse holder inside the instrument panel on the driver side - from the Fuel Pump (FP) control module (J538).






• By removing fuse, the voltage supply to terminal 30 for the FP control module is interrupted.

- Check the fuel pressure displayed.

• The fuel pressure decreases rapidly because the mechanical high pressure pump is no longer supplied with fuel from the transfer fuel pump (G6).

- Turn the ignition off as soon as the fuel pressure drops to approximately 8 bar.

• The fuel pressure must not drop below 6 bar, because otherwise the engine will shut off (risk of catalytic converter damage).


Do not let fuel come into contact with skin.

• Wear protective goggles and clothing when opening the fuel system.

• Lay clean cloths around the connection before opening the high pressure area to collect any fuel that leaks out.

- After reducing fuel pressure, place a clean cloth around the connection and open the high pressure system immediately. Escaping fuel must be absorbed.

• If the high pressure system is not opened immediately, the pressure will increase because of post-heating.

• The ignition can no longer be switched on, otherwise the pressure will increase again.