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Test Group YBMXV02.8LEV, YBMXVO4.4LEV & YBMXVO5.4LEV

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NOTE: The evaporative system monitoring permits the detection of leaks in the evaporative emission control system with a diameter of 1 mm and up.
  1. Evaporative System Monitoring General Description -  By means of a Leak Detection Pump (LDP), a vacuum actuated pump located at the atmospheric connection of the charcoal canister, a pressure test of the evaporative system is performed in the following order:
    • During the Fast Pulse Phase, the evaporative system is set under defined pressure. Pressure in the evaporative system after the Fast Pulse Phase may be higher or lower than the defined pressure depending on fuel level in the tank.
    • The Natural Frequency Phase that follows the Fast Pulse Phase, is needed to adjust the pressure to is correct value.
    • During the final Measurement Phase, the time between the pump strokes of the LOP is evaluated. In case of a time between pump strokes below a preset threshold a leak is assumed to be present.
    • During the pressure test, the purge valve needs to be shut. After the test canister purge is resumed and consequently the remaining pressure in the evaporative system is bled off.
    • To have the maximum purge capacity (or minimal loss of purge air due to leak detection) the test normally runs after engine cold start. However under certain circumstances the test may be preliminary aborted and started again later during engine run, provided all conditions for restart of the test are met.
  2. Evaporative System Monitoring Structure -  For EVAP system monitoring structure, refer to illustrations. See Fig 1-Fig 5 .
    Fig 1: EVAP System - Cold Start Conditions
    G00222232Courtesy of BMW OF NORTH AMERICA, INC.
    Fig 2: EVAP System - Conditions For Preliminary Abortion
    G00222233Courtesy of BMW OF NORTH AMERICA, INC.
    Fig 3: EVAP System - Conditions For Repetition After Preliminary Abortion
    G00222234Courtesy of BMW OF NORTH AMERICA, INC.
    Fig 4: EVAP System - Conditions For Final Abortion Of Monitoring Sequence
    G00222235Courtesy of BMW OF NORTH AMERICA, INC.
    Fig 5: EVAP System - Monitoring Sequence
    G00222236Courtesy of BMW OF NORTH AMERICA, INC.
  3. Appendix A: Use Of Ambient Temperature Signal Provided By Vehicle Can Bus -  The LDP component is specified for defined temperature range. To prevent the component from being operated under unspecified conditions, ambient temperature is used as a condition for starting the monitoring sequence after cold start.

    The ambient temperature signal is checked for circuit continuity later in the driving cycle by means of the intake air temperature monitor (TAL_REF Monitor) as follows:

    • At the time of engine start the value of TAL_REF is initialized to the maximum possible value.
    • If a specified load and a specified vehicle speed is exceeded over a specific time the value of TAL_REF is updated with the value provided by the intake air temperature sensor. This is only done if the intake air temperature value is lower than the value that is already represented by TAL_REF. Thus TAL_REF always represents the minimum value of the intake air temperature sensor found during the conditions mentioned above.

    Using this technique the following sequences are possible:

    • The ambient temperature is within specified range and no leak is detected during the monitor sequence. In this case the system is considered to be without a leak and no fault code is stored.
    • The ambient temperature is within the specified range and a leak is detected during the monitoring sequence. After conditions for updating TAL_REF are met and TAL_REF is within specified range, a fault code is stored. In case TAL_REF is out of specified range, no fault code is stored.
    • The ambient temperature is out of specified range at the time of engine start. This leads to a preliminary abortion of the monitoring sequence, the monitoring sequence is then carried out as soon as all conditions for repetition after a preliminary abortion.

  4. Appendix B: Downhill Run Detection -  The LDP measures the difference between the internal tank pressure and the ambient pressure. In the case of a vehicle moving downhill the internal tank pressure remains constant whereas the ambient pressure increases. Consequently the pressure difference measured by the LDP decreases which may cause the detection of a leak that is not present, i. e. a false alarm. To prevent this, a downhill run detection that compares the engine torque with a torque that is needed for driving the car on level terrain at a given speed, is implemented. Once a downhill run is detected the monitoring sequence is preliminary aborted.
  5. Appendix C: Typical Pressure Over Time -  For Pressure/Time graph, See Fig 6.
    Fig 6: Leak Detection Pressure Over Time Graph
    G00222237Courtesy of BMW OF NORTH AMERICA, INC.
  6. EVAP Purge System Flow Check -  Purge flow from the charcoal canister through the purge valve is monitored after the coolant temperature has reached a fixed minimum value. Diagnosis is started during regular purging.

  7. Monitoring Process

    • Step 1 (For Rich Or Lean Mixture) -  Flow through the purge valve is assumed as soon as the lambda controller is compensating for a rich or a lean shift. After this procedure the diagnosis is completed, and the evaporative purge system resumes working normally. See Fig 7.
    • Step 2 (For Stoichiometric Mixture, Or 1st Step Fails) -  In this case the lambda controller does not need to compensate for a deviation. Therefore, after finishing the regular purging operation, the purge valve is opened and closed abruptly several times. The effect of additional cylinder charge, triggers a variation of engine idle speed. If a predetermined value is reached, the diagnosis procedure is completed.

      To start diagnosis function (step 2) several conditions have to be satisfied:

      1. Vehicle speed = 0
      2. Engine at idle speed.
      3. Closed loop of lambda controller.
      4. Coolant temperature greater than a fixed limit.
      5. Transmission in any gear position except P or N.
      Furthermore, if the diagnosis has already been started and any one of the conditions has not been satisfied continuously, the process will be interrupted and started again later:
      1. No throttle position change.
      2. Engine idle speed variation greater than a fixed limit.
      3. Engine load variation less than a fixed limit.
      4. Steering wheel rotary motion less than a fixed limit.

    • Step 3 (For Stoichiometric Mixture, Or 2nd Step Fails) -  If the threshold at the 2nd step is not reached, an additional procedure is performed. The purge valve is opened and the idle air control valve is closed simultaneously to compensate the idle speed increase. The effect is a decrease of the measured idle air mass by the mass air flow sensor. If a predetermined value is reached, the diagnosis procedure is completed.
    Fig 7: EVAP System - Monitoring Structure Flow Chart
    G00222238Courtesy of BMW OF NORTH AMERICA, INC.
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