Test Group XBMXV02.8LEV

General Description -  Evaporative system monitoring permits detection of leaks in evaporative system with a diameter of .039" (1.0 mm) and up. Using Leak Detection Pump (LDP), a vacuum actuated pump located at atmospheric connection of charcoal canister, a pressure test of evaporative system is performed in following order:

• During FAST PULSE PHASE, evaporative system is set under a defined pressure. Pressure in evaporative system after FAST PULSE PHASE may be higher or lower than defined pressure depending on fuel level in tank.
• NATURAL FREQUENCY PHASE that follows FAST PULSE PHASE is therefore needed to adjust pressure to correct value.
• During FINAL MEASUREMENT PHASE, time between pump strokes of LDP is evaluated. In case a time between pump strokes is below a preset threshold, a leak is assumed to be present.

During pressure test, purge valve needs to be shut. After test, canister purge is resumed and consequently remaining pressure in evaporative system is bled off. To have maximum capacity (or minimal loss of purge air due to leak detection), test normally runs after engine cold start. However, under certain circumstances, test may be preliminary aborted and started again later during engine run if all conditions for restart of test are met.

Monitoring Structure -  See Fig 1-Fig 5 .

Use Of Ambient Temperature Signal Provided By Vehicle CAN Bus -  LDP component is specified for defined temperature range. To prevent component from being operated under unspecified conditions, ambient temperature is used as a condition for starting monitoring sequence after cold start. Ambient temperature signal is checked for circuit continuity later in driving cycle by means of intake air temperature monitor (TAL_REF Monitor):

• At time of engine start, 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, value of TAL_REF is updated with value provided by intake air temperature sensor. This is only done if intake air temperature value is lower than value that is already represented by TAL_REF. Therefore, TAL_REF always represents minimum value of intake air temperature sensor found during conditions mentioned above.

Using this technique, following sequences are possible:

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

Downhill Run Detection -  The Leak Detection Pump (LDP) measures difference between internal tank pressure and ambient pressure. In case of a vehicle moving downhill, internal tank pressure remains constant whereas ambient pressure increases. Consequently, pressure difference measured by LDP decreases, which may cause detection of a leak that is not present (false alarm). To prevent this, a downhill run detection that compares engine torque with a torque that is needed for driving car on level terrain at a given speed, is implemented. Once a downhill run is detected, monitoring sequence is preliminary aborted. See Figure and Fig 6 .

Evaporative Purge System Flow Check -  Purge flow from charcoal canister through purge valve is monitored after coolant temperature has reached a fixed minimum value. Diagnosis is started during regular purging.

Monitoring Process 

• Step 1 - For Rich Or Lean Mixture:  Flow through 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.
• Step 2 - For Stoichiometric Mixture Or Step 1 Fails:  In this case, lambda controller does not need to compensate for a deviation. Therefore, after finishing regular purging, purge valve is opened and closed abruptly several times. Effect of additional cylinder charge triggers a variation of engine idle speed. If a predetermined value is reached, diagnosis procedure is completed.
• Step 3 - For Stoichiometric Mixture Or Step 2 Fails:  If threshold at step 2 is not reached, an additional procedure is performed. Purge valve is opened and the idle air control valve simultaneously is closed to compensate idle speed increase. The effect is a decrease of measured idle air mass by mass airflow sensor. If a predetermined value is reached, diagnosis procedure is completed. See Fig 7.
  Fig 7: Monitoring Structure Of Evaporative Purge System

  Courtesy of BMW OF NORTH AMERICA, INC.