Fuel Pressure: Testing and Inspection
Fuel Pressure Regulator and Residual Pressure, Checking
Special tools, testers and auxiliary items required
• Pressure Tester (VAS 6550)
• The fuel pressure regulator regulates the fuel pressure to approximately 4 bar.
• The fuel pressure regulator is located on the fuel filter. On US vehicles, the fuel pressure regulator is integrated into the fuel filter and cannot be replaced.
• Follow all safety precautions. Refer to => [ Safety Precautions ] Fuel Supply.
• Pay attention to the rules of cleanliness. Refer to => [ Clean Working Conditions ] Fuel Supply.
The fuel supply line is under pressure! Wear protective goggles and protective clothing to prevent injuries and contact with skin. Before disconnecting the connection, wrap a cloth around the connection. Then carefully release pressure by disconnecting the connection.
- Disconnect the supply line (metal coupling) - arrow - and wipe up any draining fuel in a cloth.
• Press in the securing ring to disengage the fuel line.
- Connect the (VAS 6550) to the fuel supply line using (VAS 6550/1) and (VAS 6550/2).
- Make sure the drain is closed and the shut off valves are open.
- Start the engine and run at idle speed.
• If the engine does not start, activate the fuel pump with output Diagnostic Test Mode (DTM). Refer to => [ Function and Voltage Supply, Checking ] Testing and Inspection.
- Check the fuel pressure on the (VAS 6550). Specified value: 3.8 to 4.2 bar.
If fuel pressure is OK, check the residual pressure.
If the specification is exceeded:
- Check the fuel return line between the fuel filter and the flange for possible restrictions (kinks) or blockages.
If no malfunction can be found:
- If the fuel pressure regulator is faulty, replace it or the fuel filter.
If the specification is not obtained:
- Check the fuel lines for possible restrictions (kinks) or blockages.
If no malfunction can be found:
- Replace the fuel pressure regulator or the fuel filter.
- Repeat the test.
If the specified value is again not obtained:
- Replace the fuel delivery unit if the fuel pump is faulty. Refer to => [ Fuel Delivery Unit ] Service and Repair.
If the specified value is not reached, check the residual pressure:
- Watch the pressure decrease on the (VAS 6550). After 10 minutes the pressure must not drop below a 3.0 bar decrease.
If the pressure drops further:
- Start engine and run at idle speed.
- Turn the ignition off and close the (VAS 6550) shut off valve - B - immediately.
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 determine if there actually is a leak on the engine side.
- Check the fuel rail and fuel injectors for leaks.
If the pressure drops again:
(Leak on fuel tank side)
- Check the fuel lines for leaks.
If no malfunctions are found in the fuel lines:
- Test the check valve in the fuel pump. Refer to => [ Fuel Pump Check Valve, Checking ] Testing and Inspection.
If no malfunction is found here either:
- Replace the fuel pressure regulator or the fuel filter.
When removing the (VAS 6550) after completing the test:
- Turn off the ignition.
- Place a cloth around the connection to catch any leaking fuel.