Fuel Holding Pressure, Checking with High Pressure Pump

Fuel Holding Pressure, Checking with High Pressure Pump

Special tools, testers and auxiliary items required

• Pressure Tester (VAS 6550)

• Measuring container, fuel-resistant

• Protective gloves

Test conditions:

• Battery voltage at least 12.5 V.

• Fuel filter OK.

• Fuel tank at least 1/4 filled.

• Fuel Pump (FP) control module (J538) OK.

• The ignition is off.

Checking the Fuel Pressure


• Before opening the high pressure components of the fuel injection system, pressure must be relieved to residual pressure. Refer to => [ Releasing Pressure in High Pressure Area ] Releasing Pressure In High Pressure Area.

• Then wrap a clean cloth around the connection and relieve residual pressure by carefully loosening the connection.

- Pull the release ring - 1 - and disconnect the fuel line - 2 -,






- Connect the (VAS 6550).






- Connect a vehicle diagnostic tester.

- Select "engine electronics" in OBD.

- Select "output diagnostic test mode".

- Select "fuel pump electronics" and press "start".

• This function controls the fuel pump.

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

• Specified value: 4 to 8 bar

- Stop the function when the fuel pressure stops rising on the (VAS 6550).

If the specification is not obtained:

- Check the fuel pump delivery quantity.

Checking Holding Pressure

- 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:

- Use the output diagnostic test mode again to activate the fuel pump again in order to build fuel pressure.

- Close the shut-off valve - B - on the pressure gauge as soon as pressure accumulates. The level is perpendicular to the direction of flow.






If Pressure No Longer Drops Now:

- Check for the leak on the engine side.

- Repeat the residual pressure check. Close the shut-off valve - A - this time to determine if there actually is a leak on the engine side.

If the Pressure Drops Again:

Check for the leak on the fuel tank side.

• Check the (VAS 6550) for leaks.

• Check fuel lines and their connections for leaks.

• Check the fuel pump delivery quantity.

• Replace the fuel filter with the integrated fuel pressure regulator.

• Fuel pump check-valve is faulty.

Install in reverse order of removal. Note the following:

• Check fuel system for leaks.