Checking Fuel Pressure

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.
- 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.
- The fuel delivery pressure (low pressure) cannot be measured during a test drive. 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 control unit via the high-pressure sensor. There is no low pressure actual value.
- The fuel system has a fuel pump, which is demand-controlled by a control unit, and two supply-driven sucking jet pumps.
- The fuel system has a fuel pump, which is demand-controlled by a control unit, and two supply-driven sucking jet pumps.
- 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 --> Panamera --> DME --> Drive link test/checks --> 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 --> Panamera --> DME --> Actual values/Input signals --> B070 Fuel high pressure actual value

Fuel pressure is approx. 5500.0 hPa to 5800.0 hPa when the pump is running.







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 -->
3.1. Read out the fuel high pressure actual value in the DME control unit using the PIWIS Tester.
- 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.

Tools and materials

Test tool setup










Checking fuel pressure (low pressure) using the pressure gauge

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 vapors.
- Attach warning sign in a clearly visible position.










1. Place a cloth under the fuel supply line (low-pressure) -line 4- and open the line using two open-ended wrenches. Counter at the centre piece while doing this and loosen the union nut at the engine side. -> Fuel low-pressure line
2. Checking fuel supply pressure (low pressure) -->







2.1. Connect the test equipment. The tool setup must be fitted with sealing rings (copper, aluminium, plastic, etc.) between the individual components. The setup must be fully sealed. -> Pressure measurement connection in the vehicle
2.2. Start the engine and leave it running at idle speed. Read the fuel pressure on the pressure gauge. The pointer on the pressure gauge can fluctuate slightly at idle speed. With increased idle speed (accelerator pressed slightly), the pointer deflection calms down. The fuel low-pressure system operates at a fuel pressure of 5.5 to 5.8 bar. The pointer oscillates at around this value. Measurements taken at full throttle (with turbo in particular) can even produce values of less than 5 bar (4.5 bar) for brief periods. This is not a sign of a defect.

3. 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

3.1. Same tool setup and connection as for measuring the fuel delivery pressure (see above).
3.2. 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. Remove tool and reconnect vehicle connections -> Tightening torque: 30 Nm (22 ftlb.). Absorb and wipe off residual fuel with a cloth.