Fuel Pressure: Testing and Inspection
WM 200201 Checking fuel pressure
Tools
Technical values
Checking fuel pressure (low pressure) using the PIWIS Tester (quick test)
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
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.
Information
The fuel system contains a fuel pump, which is controlled according to demand by a control unit, and a supply-driven sucking jet pump.
Information
If you suspect that a pump in the fuel tank is defective (particularly if symptoms include a very low fuel level), always check the sucking jet pump for dirt or blockages.
Information
- Checking the low-pressure area with the high-pressure sensor cannot be used as the sole reference.
- The high-pressure sensor has a certain percent inaccuracy in the low-pressure area.
- It is best to check the low-pressure area using both test methods (Tester and pressure gauge).
General information on converting pressure =>
- 10000 hPa = 10 bar = 10000 mbar
- 1000 kPa = 10 bar = 10000 mbar
- 1 MPa = 10 bar = 10000 mbar
1. Connect the Tester and get it ready. PIWIS Tester II 9818
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. The fuel pump can be activated and deactivated as follows:
1.3. Diagnostics => Cayenne => DME => Drive link test/Tests => Drive links => Fuel pump 1
1.4. 100% is displayed. The fuel pump starts to deliver fuel when you press Start.
1.5. The fuel pump stops when the percentage value changes to 0%. Press Start again.
2. Call up the test value B070 Fuel high-pressure actual value.
2.1. Diagnostics => Cayenne => DME => Actual values/input signals => B070 Fuel high-pressure actual value
Fuel pressure is about 5800.0 hPa to 6100.0 hPa when the pump is running.
View of both PIWIS Tester values together (illustration for Panamera, German)
Information
The fuel holding pressure must only drop by max. 1 bar (1000 mbar) in one hour.
3. Step 2: Check the fuel system for leaks (check holding pressure) =>
3.1. Read out the fuel high-pressure actual value in the DME control unit using the PIWIS Tester.
- Switch on ignition, activate fuel pump briefly, then switch off the fuel pump.
- Nominal pressure is approx. 5000 to 5500.0 hPa. 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.
- The pressure is maintained. In other words: the fuel high-pressure actual value has only dropped slightly.
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 the fuse for the fuel pump.
1.3. Start the engine and wait until the engine starts up automatically.
2. Remove rear seat. -> 724819 Removing and installing rear seat - section on "Removing" 724819 Removing and installing 1/3-split folding rear seat 724819 Removing and installing 2/3-split folding rear seat
3. Remove the right cap (figure shows both caps) -arrows-. To do this, remove the carpeted floor. Then loosen the four fastening nuts and remove the cap.
Fastening nuts on caps
4. Carefully pull off the fuel supply line -arrow- at the right connection unit (place several cloths underneath to absorb the fuel that squirts out).
Fuel supply and connection points at right connection unit
5. After opening the fuel line, connect the measuring tool immediately and re-insert the fuse for the fuel pump.
Tools and materials
Test tool setup
Testing and measuring tools for fuel pressure and fuel delivery rate
Checking fuel pressure (low pressure)
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.
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, or 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 leave it running at idle speed. Read off the fuel pressure on the pressure gauge. The pressure gauge pointer can oscillate slightly at idle speed. 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 to 6.0 bar. The pointer oscillates at around this value. A measurement performed while driving by a second worker 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.
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 the fuel supply line -arrow- at the right connection unit.
Fuel supply and connection points at right connection unit
3. Fit the right cap (figure shows both caps). Then tighten the four fastening nuts. => Tightening torque: 10 Nm (7.5 ftlb.) Refit the carpeted floor and fix in place with double-sided adhesive tape.
Fastening nuts on caps
4. Install rear seat. -> 724819 Removing and installing rear seat - section on "Installing" 724819 Removing and installing 1/3-split folding rear seat 724819 Removing and installing 2/3-split folding rear seat
5. Air the vehicle with the windows open.