Fuel Pressure, Checking

Fuel Pressure, Checking

Special tools, testers and auxiliary items required

• Pressure Tester (VAS 6550)

• Vehicle Diagnostic Tester

Test Conditions

• The fuel pump function must be checked. Refer to => [ Fuel Pump, Checking ] Fuel Pump, Checking.

Test Sequence

The fuel lines are under pressure.

Danger of personal injury to eyes and skin.

Wear protective eyewear and protective clothing in order to avoid injury and contact with the skin. Wrap a cloth around the connection before removing a fuel hose. Open the connection carefully and release the pressure.

- Disconnect the fuel supply line (metal coupling) - 1 - and catch any leaking fuel with a clean cloth. Disconnect the coupling. Refer to => [ Coupling, Disconnecting ] Service and Repair.






- Connect the pressure tester (VAS 6550) to the fuel supply line using the hoses (VAS 6550/1) and (VAS 6550/2).






- Make sure the drain is closed and the shut off levers are open.

- Activate the fuel pump with output diagnostic test mode to build fuel pressure.

- Read the fuel pressure.

Specified Value:

• Engine code CDVB: 4.0 to 7.0 bar

• Engine codes CBTA and CBUA: 3.5 to 4.5 bar

If the fuel pressure is OK, check the residual pressure. Refer to => [ Holding Pressure, Checking ] Holding Pressure, Checking.

If the specification is exceeded:

- The pressure relief valve inside the fuel delivery unit flange is faulty.

• The pressure relief valve and the fuel filter are integrated in the fuel delivery unit flange and cannot be replaced separately.

- Replace the fuel delivery unit flange and fuel filter. Refer to => [ Fuel Delivery Unit/Fuel Level Sensor ] Fuel Delivery Unit/Fuel Level Sensor.

If the specification is not obtained:

- Check the fuel lines for possible restrictions (kinks) or blockages.

- Remove the fuel delivery unit and inspect it for debris. Refer to => [ Fuel Delivery Unit/Fuel Level Sensor ] Fuel Delivery Unit/Fuel Level Sensor.

- Make sure all the lines - arrows - are connected.






If no malfunction can be found:

- Replace the fuel delivery unit flange.

- Repeat the test.

If the specification is again not obtained:

- The fuel pump is faulty, replace the fuel delivery unit. Refer to => [ Fuel Delivery Unit/Fuel Level Sensor ] Fuel Delivery Unit/Fuel Level Sensor.