Fuel Pressure and Residual Pressure Before High Pressure Pump, Checking

Fuel Pressure and Residual Pressure before High Pressure Pump, Checking

Special tools, testers and auxiliary items required

• Pressure Tester (VAS 6550) with Hose (VAS 6550/1) and (VAS 6550/2)

• Measuring container, fuel-resistant

Test Conditions

• Battery voltage at least 12.5 V.

• Fuel filter OK.

• Fuel tank at least 1/4 full.

• Fuel Pump (FP) control module (J538) OK; checking

• The ignition is off.

Test Sequence


The fuel system is pressurized (fuel supply pressure). Wear protective goggles and protective clothing to prevent injuries and contact with skin. Place rags around the connection before opening the system. Then reduce residual pressure by carefully removing connection.

- Disconnect the fuel supply line - arrow - by pulling the release button.






- Connect the (VAS 6550) to the fuel supply line with the (VAS 6550/1) and (VAS 6550/2).






- Make sure the drain valve (hose in the center) is closed and the shut-off valves (left and right) are open.

- Turn the ignition on.

- The fuel pump runs for approximately 5 seconds and the fuel pressure builds.

- Read the fuel pressure on the (VAS 6550).

• Specified value: 4.0 to 7.0 bar

If the specification is not obtained:

- Check fuel pump delivery rate.

Residual Pressure, Checking

- Check for leaks and for residual pressure by observing the pressure loss on the (VAS 6550).

• After 10 minutes there must be a residual pressure of at least 3 bar.

If the residual pressure drops below 3 bar:

• Check the threaded connection between the (VAS 6550) and fuel supply line for leaks.

• Check (VAS 6550) for leaks.

• Check fuel lines and their connections for leaks.

• Replace fuel filter with integrated fuel pressure regulator. Then repeat test.

• If the fuel filter is OK, replace fuel pump.

Assembly is performed in the reverse order of removal, noting the following:

- The ignition must be off.

• Open the drain valve (hose in the center) and release the fuel pressure before removing the pressure tester. Hold the hose in a measuring container while doing this.






- Connect the fuel supply line - arrow - (make sure it is clean and there are no leaks).