Fuel Pressure: Testing and Inspection
WM 200201 Checking fuel pressure (low pressure) on DFI engines
Tools
Checking fuel pressure (low pressure) on DFI engines using the PIWIS Tester
Information
- 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.
Information
The latest version of the procedure for checking the low-pressure area can be found under Guided Fault Finding on the PIWIS Tester.
Information
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 206619 Removing and installing fuel pump (in-tank fuel pump)
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 less than 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 =>
Information
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
1. Step 1: Check the fuel system for leaks =>
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.).
1.2. Connect the PIWIS Tester and call up DME actual values.
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.
1.4. Switch on ignition.
1.5. Wait for about 1 minute until the fuel high pressure has dropped to its holding pressure.
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.
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.
2.1. Switch off the ignition.
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.
3. Step 3: Relieve the fuel pressure =>
3.1. Remove fuses 13 + 14 from the E-box (plenum panel).
3.2. Start the engine and wait until the engine starts up automatically. => Continue with Step 4.
4. Step 4: Check fuel pump 2 (right). =>
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.
4.2. Switch off ignition.
4.3. Remove relay 19 for fuel pump 2 (right).
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).
4.5. Switch on ignition.
4.6. Select the fuel high pressure actual value in the DME (DFI) control unit using the PIWIS Tester.
4.7. Install an ammeter and insert the correct fuse in the slot for fuse 13.
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.
4.9. Remove the ammeter and fuse from the slot for fuse 13 and remove auxiliary line from the relay socket.
5. Step 5: Check relay for fuel pump 2 (right) =>
5.1. Install the correct fuse in the slot for fuse 13.
5.2. Install relay 19 for fuel pump 2 (right).
5.3. Switch on ignition.
5.4. Select the fuel high pressure actual value using the PIWIS Tester.
5.5. Start the engine.
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.
6. Step 6: Relieve the fuel pressure =>
6.1. Remove fuses from the slot for fuse 13 and 14 in the fuse box (plenum panel).
6.2. Start the engine and wait until the engine starts up automatically.
6.3. Continue with Step 7.
7. Step 7: Check fuel pump 1 (left) =>
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.
7.2. Switch off ignition.
7.3. Remove relay 6 for fuel pump 1 (left).
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).
7.5. Switch on ignition.
7.6. Select the fuel high pressure actual value in the DME (DFI) control unit using the PIWIS Tester.
7.7. Install an ammeter and insert the correct fuse in the slot for fuse 14.
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.
7.9. Remove the ammeter and fuse from the slot for fuse 14 and remove auxiliary line from the relay socket.
8. Step 8: Check relay for fuel pump 1 (left) =>
8.1. Install the correct fuse in the slot for fuse 14.
8.2. Install relay 6 for fuel pump 1 (left).
8.3. Switch on ignition.
8.4. Select the fuel high pressure actual value using the PIWIS Tester.
8.5. Start the engine.
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.
9. Step 9: Continue checking the fuel high-pressure system (see below).
10. Step 10: Check flow and electric circuit for fuel pump 2 (right) =>
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.
10.2. Check relay for fuel pump 2 (right).
10.3. Check electric circuit, including connectors and lines, between fuse 13 and the relay.
10.4. Check electric circuit, including connection points, connectors and lines, between the relay and fuel pump 2 (right).
10.5. Check fuel pump 2 (right).
10.6. Check that fuel lines and filters are not blocked.
10.7. Repair or replace defective component.
10.8. Repeat check, starting at Step 3.
11. Step 11: Check flow and electric circuit for fuel pump 1 (left) =>
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.
11.2. Check relay for fuel pump 1 (left).
11.3. Check electric circuit, including connectors and lines, between fuse 14 and the relay.
11.4. Check electric circuit, including connection points, connectors and lines, between the relay and fuel pump 1 (left).
11.5. Check fuel pump 1 (left).
11.6. Check that fuel lines and filters are not blocked.
11.7. Repair or replace defective component.
11.8. Repeat check, starting at Step 6.
12. 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.
12.1. Fit a test pressure gauge at the connecting flange. For details, see below.
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.
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.
13. Step 13: Check fuel delivery rate (low-pressure delivery rate) on DFI engines =>
13.1. -> 206601 Checking fuel delivery rate (low-pressure delivery rate) on DFI engines Testing and Inspection
14. Step 14: Check the fuel high-pressure system using the PIWIS Tester =>
14.1. Fault entered in fault memory => Proceed according to Guided Fault Finding.
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. -> 246319 Removing and installing DFI high-pressure pump 246319 Removing and installing DFI high-pressure pump
- The actual value is consistent with the nominal value. No faults can be found.
Preliminary work
WARNING
Toxic substances
- Danger of poisoning or suffocation
-> Ventilate the work area well.
-> Never ingest or inhale.
-> Read specific information on the Hazard Sheet.
-> Wear personal protective gear.
WARNING
Highly inflammable or explosive materials
- Danger of fire or explosion
-> Avoid contact with hot parts or sources of ignition.
-> Use suction to remove ignitable vapours.
-> Attach warning sign in a clearly visible position.
Information
- Absorb any escaping fuel using a suitable binding material or a cloth.
- Cloths which have absorbed fuel must be disposed of according to the applicable national laws.
- The fuel system must be depressurised before starting work on it.
- Only work on the fuel system in well-ventilated rooms.
- Any seals which have been removed must always be replaced.
- Do not use any sealant containing silicone or silicone spray when the injection system is open. Traces of silicone drawn in by the engine are not combusted and damage the oxygen sensors.
1. Relieve the fuel pressure =>
1.1. Switch off ignition.
1.2. Pull out fuse 13 and 14.
1.3. Start the engine and wait until the engine cuts out on its own accord.
1.4.
2. Loosen rear seat and fold it up. Secure to the headrest using a piece of wire if necessary.-> 724819 Removing and installing rear seat - section on "Removing" 724819 Removing and installing rear seat (single-seat look)
3. Loosen reinforcement plate for right seat-belt buckle. To do this, loosen the three fastening screws and twist the plate with the seat-belt buckle to the side. Do not use the fastening screws again.
4. Loosen the right cap (figure shows both) -arrows-. To do this, remove the carpeted floor. Then unscrew the four fastening nuts and remove the cap.
Fastening nuts for cap
5. Carefully detach the fuel supply line -1- at the right connection unit (place several cloths underneath to absorb the fuel that squirts out).
Fuel supply and disconnection points at right connection unit
6. After opening the fuel line, connect the measuring tool immediately and re-insert the fuses for the fuel pumps.
Tools and materials
Tools for checking fuel pressure and fuel delivery rate
Testing and measuring tools for fuel pressure and fuel delivery rate
1. Adapting adapter 9479 =>
1.1. The adapter 9479 -2- must be adapted to suit the slots on terminal 30 and 87 of the fuel pump relay using two short auxiliary lines -1-. The auxiliary lines are shop-made or lines from a suitable measuring equipment range can be used.
Auxiliary cable to adapter
1.2. Disconnect the fuel pump relays for both fuel pumps in the central electrical system. Instead of the contacts on terminal 30 and 87 of the relay, use the two contacts on the auxiliary lines to activate the fuel pump (left or right). => Terminals 30 and 87
Terminals 30 and 87
1.3. Select the correct slots for the left or right fuel pump. -1 and 2-
Slots for the fuel pump relays
1.4. Adapter 9479 must be inserted at the contacts using auxiliary lines. The left or right fuel pump in the tank can now be activated, depending on the slot.
Adapter inserted
Checking fuel pressure (low pressure)
1. Checking fuel supply pressure (low pressure) =>
1.1. Connect pressure gauge with stopcock tester K-Jetronic Tester V.A.G. 1318 WE 1291 using adapter 1318/7 and connecting line connection line 9705/3 ("female" rapid-action coupling) on the side of the stopcock. On the other side of the stopcock, connect adapter 1318/15 (one-piece connecting piece) and connecting line connection line 9705/3 ("male" rapid-action coupling). The tool setup must be fitted with sealing rings (copper, aluminium, plastic, etc.) between the individual components. The setup must be fully sealed. => Measuring tool for fuel delivery pressure
Measuring tool for fuel delivery pressure
1.2. Connect measuring tool -2- between the connection unit output -3- and the fuel supply line -1-. The stopcock is set to flow.
Tool setup connected in vehicle
1.3. Start the engine and allow it to run at idle speed. Read the fuel pressure on the pressure gauge. The pointer on the pressure gauge may oscillate when idling (due to the high-pressure pump switching between pressure stage and intake stage). With increased idle speed (accelerator pressed slightly), the pointer deflection calms down. The fuel low-pressure system operates at a constant pressure of 5.5 bar. The pointer oscillates at around this value. A measurement performed while driving by a second person can also generate values of below 5 bar (4.5 bar) briefly at full throttle (with turbo in particular). This is not a sign of a defect.
1.4. Alternatively, each individual fuel pump in the tank can also be actuated using the adapter 9479 instead of the fuel pump relay. For assignment of fuel pump relays - see wiring diagram. Each individual fuel pump generates a constant pressure of 5.5 bar.
2. Checking fuel retaining pressure (low pressure) =>
- Engine cold
- Workshop air-conditioned to normal temperature
- Fluctuations may occur if the engine is warm or the outside temperature is high
- Fully sealed measuring tool setup
2.1. Same tool setup and connection as for measuring the fuel delivery pressure (see above).
2.2. Allow engine to run for a few minutes. Switch off engine. Check the pressure drop. Drop in fuel pressure after "ignition is switched off" must not exceed 1 bar after 1 hour. Make sure that the tool setup itself does not have any leaks (even the slightest leak). The fuel retaining pressure will decrease extremely rapidly and falsify the measured result with even the tiniest leak.
Subsequent work
1. Disconnect tool and reconnect vehicle connections. Absorb and wipe off residual fuel with a cloth.
2. Fit cover plate.
3. Fit seat belt.
4. Fit rear seat.
5. Air the vehicle with the windows open.