Checking Fuel Pressure (Low Pressure) On Dfi Engines Using The Piwis Tester

WARNING: This page is about a different car, the 2010 Porsche Cayenne, 2009 Porsche Cayenne, and 2008 Porsche Cayenne. However, it is still accessible from the selected car via links, so may be relevant.

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Faults in the fuel high-pressure injection system can also be caused by defects in the pre-delivery system (fuel low pressure).
In the event of problems in the fuel high-pressure injection system, the pre-delivery system should be checked first.

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The latest version of the procedure for checking the low-pressure area can be found under Guided Fault Finding on the PIWIS Tester.

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The fuel delivery pressure (low pressure) can only be measured during a test drive when a pressure gauge is connected. When you drive the vehicle, only the measured value Fuel high pressure actual value can be read out with the PIWIS Tester in the DME (DFI) control unit via the high-pressure sensor. There is no "low pressure" actual value. This must be checked with the engine running or during a journey using a pressure gauge.

Information

If you suspect that the pumps in the fuel tank are defective (particularly if symptoms include a very low fuel level), always check the sucking jet pumps for dirt or blockages. A piping diagram and assembly overview are shown in the information section for the fuel pumps. → 206619 REMOVING AND INSTALLING FUEL PUMP (IN-TANK FUEL PUMP) ON DFI ENGINES

Strategy for fuel pump operation =>

The fuel pumps in the tank are activated differently depending on the fuel level in the tank and the load. For details, see also the PIWIS TESTER under Guided Fault Finding => Cayenne, 2008 => Function descriptions (animations) => Tank system
Both fuel pumps are activated when you start the engine and for up to 10 seconds afterwards.
Fuel pump 2 (right) is always activated when the fuel level in the tank is > 60 litres.
Fuel pump 1 (left) is always activated when the fuel level in the tank is > 15 litres and < 60 litres.
Both fuel pumps are always activated when the fuel level in the tank is < 10 litres.
There is a hysteresis when the fuel level in the tank is between 10 and 15 litres. In other words: A) if the fuel level in the tank is falling, fuel pump 2 (right) is activated when there is < 10 litres of fuel in the tank, B) if the fuel level in the tank is increasing, fuel pump 2 (right) is deactivated when there is > 15 litres of fuel in the tank.
If a lot of fuel is required (> 60 l/h), both fuel pumps are activated, e.g. for accelerating at full throttle in 3rd gear, engine speed > 2500 rpm + load > 50 %, mass air flow > 350 kg/h or > 200 mg/stroke.

Analysing the extended fault memory =>

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If a fault is stored in the DFI control unit, the extended fault memory in the PIWIS Tester must be examined for further diagnosis.

Fuel level in the tank when the fault occurs for the first time => see Fault memory under F8 => EXTENDED FAULT MEMORY OBD Freeze Frame => Value Fuel Level Input (the value shows the fuel level as a % of the total tank volume, e.g. 79 % means a fuel level of 79 litres of a total tank volume of 100 litres.)

Determining dependencies between the fuel level in the tank and faults that have occurred =>

Does the fault/symptom occur at a specific fuel level? If it does: only at fuel levels of > 60 litres => Continue with Steps 1 to 4.
Does the fault/symptom occur at a specific fuel level? If it does: only at fuel levels 10... 60 litres = > Continue with Steps 1 to 3 and Step 7.
Does the fault/symptom only occur at high rev/load ranges? If it does: => Continue with Steps 1 + 13.

General information on converting pressure =>

10000 hPa = 10 bar = 10000 mbar
1000 kPa = 10 bar = 10000 mbar
1 MPa = 10 bar = 10000 mbar

Step 1: Check the fuel system for leaks =>
1. 1.1. Connect a charger to the vehicle battery. The voltage should be approx. 12.5 V (no rapid charging allowed because the voltage is too high!).
2. 1.2. Connect the PIWIS Tester and call up DME actual values.
3. 1.3. Start the engine and then stop it again.
  Information
  
  The ignition must be switched on using the vehicle key.
  - If the ignition is switched on using the control panel, it shuts down automatically after about 10 minutes.
4. 1.4. Switch on ignition.
5. 1.5. Wait for about 1 minute until the fuel high pressure has dropped to its holding pressure.
6. 1.6. Read out the fuel high pressure actual value in the DME (DFI) control unit using the PIWIS Tester.
  Information
  
  The fuel holding pressure must only drop by max. 1 bar (1000 mbar) in one hour.
  - Nominal pressure is approx. 5000.0 mbar to 5500.0 mbar => see Actual values => [F5] => Fuel high-pressure control system => Select value. Monitor the fuel pressure for about 5 to 10 minutes.
  - The pressure is not maintained. In other words: the fuel high pressure actual value has dropped considerably. There is an internal or external leak in the fuel system. => Continue with Step 12.
  - The pressure is maintained. In other words: the fuel high pressure actual value has only dropped slightly. => Continue with Step 2 .
Step 2: Check fuses =>
Information

If fuses are faulty, the two Driver for fuel pump relay faults (P0230 + P2632) will be entered in the DME (DFI) fault memory.
1. 2.1. Switch off the ignition.
2. 2.2. Remove fuses 13 + 14 from the E-box (plenum panel) and check them: =>
  - Fuse 13 is faulty => Continue with Step 10.
  - Fuse 14 is faulty => Continue with Step 11.
  - Both fuses are OK => Continue with Step 3 .
Step 3: Relieve the fuel pressure =>
1. 3.1. Remove fuses 13 + 14 from the E-box (plenum panel).
2. 3.2. Start the engine and wait until the engine starts up automatically. => Continue with Step 4 .
Step 4: Check fuel pump 2 (right). =>
1. 4.1. The next test step checks whether the fuel pump builds up enough admission pressure at the correct power consumption level. The fuel pump starts up when the fuse is inserted.
2. 4.2. Switch off ignition.
3. 4.3. Remove relay 19 for fuel pump 2 (right).
4. 4.4. Bridge (join) the contacts for terminal 30 and terminal 87 using a suitable auxiliary line on the relay socket for fuel pump 2 (right).
5. 4.5. Switch on ignition.
6. 4.6. Select the fuel high pressure actual value in the DME (DFI) control unit using the PIWIS Tester.
7. 4.7. Install an ammeter and insert the correct fuse in the slot for fuse 13.
8. 4.8. Check the power consumption of the fuel pump. => Nominal current is max. 13 A. Read out the fuel high pressure actual value in the DME (DFI) control unit using the PIWIS Tester. Nominal pressure is approx. 5000.0 mbar to 5500.0 mbar.
  - Fuel high pressure actual value is significantly less than 5000.0 mbar or power consumption of the fuel pump is significantly greater than 13 A => Continue with Step 10.
  - Fuel high pressure actual value is greater than 5000.0 mbar and power consumption of the fuel pump is less than 13 A => Continue with Step 5.
9. 4.9. Remove the ammeter and fuse from the slot for fuse 13 and remove auxiliary line from the relay socket.
Step 5: Check relay for fuel pump 2 (right) =>
1. 5.1. Install the correct fuse in the slot for fuse 13.
2. 5.2. Install relay 19 for fuel pump 2 (right).
3. 5.3. Switch on ignition.
4. 5.4. Select the fuel high pressure actual value using the PIWIS Tester.
5. 5.5. Start the engine.
6. 5.6. Read out the fuel high pressure actual value using the PIWIS Tester. Nominal pressure is greater than 5000.0 mbar.
  - The fuel high pressure actual value is significantly less than 5000.0 mbar. Replace relay for fuel pump 2 (right). Repeat check, starting at Step 3.
  - The fuel high pressure actual value is greater than 5000.0 mbar. => Continue with Step 6 .
Step 6: Relieve the fuel pressure =>
1. 6.1. Remove fuses from the slot for fuse 13 and 14 in the fuse box (plenum panel).
2. 6.2. Start the engine and wait until the engine starts up automatically.
3. 6.3. Continue with Step 7 .
Step 7: Check fuel pump 1 (left) =>
1. 7.1. The next test step checks whether the fuel pump builds up enough admission pressure at the correct power consumption level. The fuel pump starts up when the fuse is inserted.
2. 7.2. Switch off ignition.
3. 7.3. Remove relay 6 for fuel pump 1 (left).
4. 7.4. Bridge (join) the contacts for terminal 30 and terminal 87 using a suitable auxiliary line on the relay socket for fuel pump 1 (left).
5. 7.5. Switch on ignition.
6. 7.6. Select the fuel high pressure actual value in the DME (DFI) control unit using the PIWIS Tester.
7. 7.7. Install an ammeter and insert the correct fuse in the slot for fuse 14.
8. 7.8. Check the power consumption of the fuel pump. => Nominal current is max. 13 A. Read out the fuel high pressure actual value in the DME (DFI) control unit using the PIWIS Tester. Nominal pressure is approx. 5000.0 mbar to 5500.0 mbar.
  - Fuel high pressure actual value is significantly less than 5000.0 mbar or power consumption of the fuel pump is significantly greater than 13 A => Continue with Step 11.
  - Fuel high pressure actual value is greater than 5000.0 mbar and power consumption of the fuel pump is less than 13 A => Continue with Step 8.
9. 7.9. Remove the ammeter and fuse from the slot for fuse 14 and remove auxiliary line from the relay socket.
Step 8: Check relay for fuel pump 1 (left) =>
1. 8.1. Install the correct fuse in the slot for fuse 14.
2. 8.2. Install relay 6 for fuel pump 1 (left).
3. 8.3. Switch on ignition.
4. 8.4. Select the fuel high pressure actual value using the PIWIS Tester.
5. 8.5. Start the engine.
6. 8.6. Read out the fuel high pressure actual value using the PIWIS Tester. Nominal pressure is greater than 5000.0 mbar.
  - The fuel high pressure actual value is significantly less than 5000.0 mbar. Replace relay for fuel pump 1 (left). Repeat check, starting at Step 6.
  - The fuel high pressure actual value is greater than 5000.0 mbar. => Continue with Step 9 .
Step 9: Continue checking the fuel high-pressure system (see below).
Step 10: Check flow and electric circuit for fuel pump 2 (right) =>
1. 10.1. Check that connectors, plug connections, contacts, lines and components are seated securely, are connected and show no signs of damage, oxidation or corrosion.
2. 10.2. Check relay for fuel pump 2 (right).
3. 10.3. Check electric circuit, including connectors and lines, between fuse 13 and the relay.
4. 10.4. Check electric circuit, including connection points, connectors and lines, between the relay and fuel pump 2 (right).
5. 10.5. Check fuel pump 2 (right).
6. 10.6. Check that fuel lines and filters are not blocked.
7. 10.7. Repair or replace defective component.
8. 10.8. Repeat check, starting at Step 3.
Step 11: Check flow and electric circuit for fuel pump 1 (left) =>
1. 11.1. Check that connectors, plug connections, contacts, lines and components are seated securely, are connected and show no signs of damage, oxidation or corrosion.
2. 11.2. Check relay for fuel pump 1 (left).
3. 11.3. Check electric circuit, including connectors and lines, between fuse 14 and the relay.
4. 11.4. Check electric circuit, including connection points, connectors and lines, between the relay and fuel pump 1 (left).
5. 11.5. Check fuel pump 1 (left).
6. 11.6. Check that fuel lines and filters are not blocked.
7. 11.7. Repair or replace defective component.
8. 11.8. Repeat check, starting at Step 6.
Step 12: Check leak site in the fuel system =>
Information

The fuel holding pressure must only drop by max. 1 bar (1000 mbar) in one hour.
1. 12.1. Fit a test pressure gauge at the connecting flange. For details, see below.
2. 12.2. Check holding pressure of fuel low-pressure system (check for leaks). => Pressure is not maintained. => Check lines between the fuel pumps and high-pressure pump for leaks. Check the pressure regulator. Check the check valves on both fuel pumps. Repair or replace defective component.
  Information
  
  A leak in the fuel high-pressure pump supplying the engine (mechanical drive) dilutes the oil. Check the engine oil and initiate appropriate remedial action if necessary.
3. 12.3. Check holding pressure of fuel low-pressure system (check for leaks). => Pressure is maintained. => There are no leaks in the pre-delivery system. Check fuel high-pressure system for leaks. Perform a visual inspection of the high-pressure rail. Perform an endoscopic visual inspection into the cylinders (injector leak). Check for leaks in the fuel high-pressure pump supplying the engine (mechanical drive). Repair or replace defective component.
Step 13: Check fuel delivery rate (low-pressure delivery rate) on DFI engines =>
1. 13.1. → 206601 CHECKING FUEL DELIVERY RATE (LOW-PRESSURE DELIVERY RATE) ON DFI ENGINES
Step 14: Check the fuel high-pressure system using the PIWIS Tester =>
1. 14.1. Fault entered in fault memory => Proceed according to Guided Fault Finding .
2. 14.2. No faults entered in fault memory => Get another person to compare the fuel high-pressure nominal value with the fuel high pressure actual value during a test drive. Under Actual values, press [F5] to select the actual value Fuel high-pressure control system. [F3] displays the data logger .
  - The actual value is considerably different to the nominal value. => Replace the high-pressure pump and flow control valve. → WM 246319 REMOVING AND INSTALLING DFI HIGH-PRESSURE PUMP
  - The actual value is consistent with the nominal value. No faults can be found.