Fuel Pressure: Testing and Inspection
Fuel Pump, Checking
=> [ Fuel Pressure, Checking ]
=> [ Residual Pressure, Checking ]
=> [ Fuel Delivery, Checking ]
Special tools, testers and auxiliary items required
• Pressure Tester (VAS 6550)
• Adapter Set for VAS 6550 (VAS 6550/3)
• Vehicle Diagnostic Tester
• Measuring Container
• Wiring Diagram
Test Conditions:
• The Engine Control Module (ECM) Diagnostic Trouble Code (DTC) memory does not contain a malfunction related to the voltage supply of the fuel pump.
• The fuel pump control module is OK
• The battery charger is connected.
• The fuel pump fuse is OK.
• The fuel filter is OK.
• The fuel tank is filled at least above the reserve.
• The ignition is turned off.
Fuel Pressure, Checking
- Read the safety precautions before starting. Refer to => [ Safety Precautions ] Safety Precautions.
- Follow the guidelines for cleanliness. Refer to => [ Clean Working Conditions ] Clean Working Conditions.
- Release the high side fuel pressure. Refer to => [ High Side Fuel Pressure, Releasing ] Service and Repair.
The fuel lines are under pressure.
Danger of personal injury to eyes and skin.
Wear protective goggles 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 hose - arrow - from the fuel supply line.
- Connect the pressure tester (VAS 6550) to the hose (VAS 6550/2) , the connection (VAS 6550/3-1) and the hose (VAS 6550/3-2) as shown.
• Secure the hoses with the included clamps.
• Ensure proper seating of the hose clamps.
- Open the shut off levers "A" and "B" on the pressure tester (VAS 6550) in the direction of flow.
- Close the shut off lever "C".
- Press the ON/OFF button to turn on the pressure tester (VAS 6550).
- Turn the ignition on.
- Connect the vehicle diagnostic tester and conduct the guided function "activate fuel pump".
• The fuel pump is activated.
• The specified value will not be reached if the fuel pressure is measured with the "Guided Function" "start fuel pump".
• The specified value will not be reached if the fuel pressure is measured without the battery charger connected.
- Read off the pressure on the pressure tester (VAS 6550). Specified value: Greater than 5.5 bar.
- Slowly close the shut off lever "B" on the pressure tester (VAS 6550).
- Read the pressure on the pressure tester (VAS 6550). Specified value: Greater than 5.5 bar.
- Turn off the ignition.
If the specified value is achieved:
- Check the residual pressure. Refer to => [ Residual Pressure, Checking ].
If the specified value is not obtained:
• If the pressure tester (VAS 6550) is to be removed, open the shut off lever "C" and reduce the residual pressure. For this purpose, hold the hose from the shut off lever "C" in a measuring container.
- Remove the pressure tester (VAS 6550) and reconnect the fuel supply line.
- Make sure the hose clamp is securely fastened.
- Remove the rear seat bench.
- Fold the right carpet cutout to the side.
- Loosen the nuts - 1 through 4 - for the flange cover.
- Remove the flange cover.
- Disconnect the fuel supply line - 2 -.
• Press the button on the line coupling together for this purpose.
- Connect the pressure tester (VAS 6550) to the hose (VAS 6550/3-4) and the hose (VAS 6550/3-3) as shown.
- Open the shut off levers "A" and "B" on the pressure tester (VAS 6550) in the direction of flow.
- Close the shut off lever "C".
- Turn the ignition on.
- Perform the "Guided Function" "Activate Fuel Pump" again.
• The fuel pump is activated.
- Read off the pressure on the pressure tester (VAS 6550). Specified value: Greater than 5.5 bar.
- Slowly close the shut off lever "B" on the pressure tester (VAS 6550).
- Read the pressure on the pressure tester (VAS 6550). Specified value should be greater than 5.5 bar.
- Turn off the ignition.
If the target values are achieved:
- Check the fuel supply line from the fuel tank to the engine for possible damage.
If specifications are again not obtained:
- Make sure all couplings and fuel lines in the fuel tank and the fuel delivery unit are secure and not leaking.
If no malfunction can be found:
- Replace the fuel delivery unit. Refer to => [ Fuel Delivery Unit and Fuel Level Sensor ] Fuel Delivery Unit and Fuel Level Sensor.
• If the pressure tester (VAS 6550) is to be removed, open the shut off lever "C" and reduce the residual pressure. For this purpose, hold the hose from the shut off lever "C" in a measuring container.
Residual Pressure, Checking
Test Conditions:
• The pressure tester (VAS 6550) remains connected. Refer to => [ Fuel Pressure, Checking ].
• Fuel pressure approximately 5.0 bar
Test Sequence:
- Observe the pressure drop on the pressure gauge. After 10 minutes the pressure must not drop below a 4.3 bar decrease.
If the pressure drops below 4.3 bar:
- Turn the ignition on.
- Perform the "Guided Function" "Activate Fuel Pump" again.
• The fuel pump is activated.
The fuel pressure must increase up to a maximum pressure of greater than 5.5 bar.
- Turn on the ignition and immediately close the shut off lever "B" on the pressure tester. Shut off lever "B" points perpendicular to the direction of flow.
- Observe the pressure drop on the pressure tester. After 10 minutes the pressure must not drop below a 4.3 bar decrease.
If the pressure drops below 4.3 bar again:
(The leak is on fuel tank side)
- Check the fuel tank connections for leaks.
If no malfunction can be found:
- Check the fuel line to the flange for leaks.
- Check all additional fuel lines in fuel tank for leaks.
If no malfunctions are found on fuel line:
- Replace the fuel delivery unit. Refer to => [ Fuel Delivery Unit and Fuel Level Sensor ] Fuel Delivery Unit and Fuel Level Sensor.
• If the pressure tester (VAS 6550) is to be removed, open the shut off lever "C" and reduce the residual pressure. For this purpose, hold the hose from the shut off lever "C" in a measuring container.
Fuel Delivery, Checking
- Read the safety precautions before starting. Refer to => [ Safety Precautions ] Safety Precautions.
- Follow the guidelines for cleanliness. Refer to => [ Clean Working Conditions ] Clean Working Conditions.
- Release the high side fuel pressure. Refer to => [ High Side Fuel Pressure, Releasing ] Service and Repair.
Test Sequence:
• A second mechanic is needed for the following task.
The fuel lines are under pressure.
Danger of personal injury to eyes and skin.
Wear protective goggles 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 hose - arrow - from the fuel supply line.
- Connect the pressure tester (VAS 6550) to the hose (VAS 6550/2) , the connection (VAS 6550/3-1) and the hose (VAS 6550/3-2) as shown.
- Hold the other end of the hose (VAS 6550/3-2) in a measuring container - 1 -.
- Open the shut off levers "A" and "B" on the pressure tester (VAS 6550) in the direction of flow.
- Close the shut off lever "C".
- Press the ON/OFF button to turn on the pressure tester (VAS 6550).
- Turn the ignition on.
- Perform the "Guided Function" "Activate Fuel Pump".
• The fuel pump is activated.
- Slowly close the shut off lever - B - until a counterpressure of 4 bar is achieved.
- Turn off the ignition.
- Empty the measuring container.
- Perform the "Guided Function" "Activate Fuel Pump" again.
• The fuel pump is activated.
- Turn off the ignition after 30 seconds.
• Minimum delivery rate specified value: Greater than 1.5L/30 seconds.
If the target value is not reached, the measurement at the fuel delivery unit flange must be repeated:
• If the pressure tester (VAS 6550) is to be removed, open the shut off lever "C" and reduce the residual pressure. For this purpose, hold the hose from the shut off lever "C" in a measuring container.
- Remove the pressure tester (VAS 6550) and reconnect the fuel supply line.
- Make sure the hose clamp is securely fastened.
- Remove the rear seat bench.
- Fold the right carpet cutout to the side.
- Loosen the nuts - 1 through 4 - for the flange cover.
- Remove the flange cover.
- Disconnect the fuel supply line - 2 -.
• Press the button on the line coupling together for this purpose.
- Connect the pressure tester (VAS 6550) to the hose (VAS 6550/3-3) and the hose (VAS 6550/3-4) as shown.
- Hold the end of the hose (VAS 6550/3-3) in a measuring container.
- Open the shut off levers "A" and "B" for the pressure tester (VAS 6550) in the direction of flow.
- Close the shut off lever "C".
- Turn the ignition on.
- Perform the "Guided Function" "Activate Fuel Pump".
• The fuel pump is activated.
- Slowly close the shut off lever - B - until a counterpressure of 4 bar is achieved.
- Turn off the ignition.
- Empty the measuring container.
- Turn the ignition on.
- Perform the "Guided Function" "Activate Fuel Pump" again.
• The fuel pump is activated.
- Turn off the ignition after 30 seconds.
• Minimum delivery rate specified value: Greater than 1.5L/30 seconds.
- Turn off the ignition.
If the specification is now obtained:
- Check the fuel supply line to the engine for kinks and damage.
If specified value is again not obtained:
- Replace the fuel filter. Refer to => [ Fuel Filter ] Service and Repair.
- Repeat the test.
If specified value is again not obtained:
- The fuel pump is faulty, replace the fuel delivery unit. Refer to => [ Fuel Delivery Unit and Fuel Level Sensor ] Fuel Delivery Unit and Fuel Level Sensor.
• If the pressure tester (VAS 6550) is to be removed, open the shut off lever "C" and reduce the residual pressure. For this purpose, hold the hose from the shut off lever "C" in a measuring container.