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Releasing Pressure In High Pressure Area






Releasing Pressure in High Pressure Area

Special tools, testers and auxiliary items required

Special Hook (3370)


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 6 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.

High Fuel Pressure, Reducing

- Connect a vehicle diagnosis tester.

- Start engine and run at idle speed.

- Select "engine electronics" in OBD.

- Then select "Measured values" read out.

- Select measured value block 140.

- With the engine running in idle, the fuel pressure will be displayed in display field 3.

Specified value: between 35 and 45 bar

- Pry off the left cover on the instrument panel using (3370) - 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).






- Check the fuel pressure on the tester.

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.

The fuel lines will continue to be filled with fuel, but will no longer be under high pressure. Wear protective goggles and protective clothing when opening the fuel system.

Before opening the high pressure system, place a cloth around the connection.

- 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.

Final Procedures

- Install the fuse.

- Erase the Diagnostic Trouble Code (DTC) memory and generate the readiness code in the Engine Control Module (ECM) in "Guided Functions".