Test Group VBM2.8VJGKEK

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Evaporative System Monitoring General Description - The evaporative system monitoring permits the detection of leaks in the evaporative emission control system with a diameter of 1 mm and up. See Figure.

The evaporative emission control system is hermetically sealed off from the atmosphere by means of a shut-off valve at the charcoal canister. When the purge valve opens, the engine generates a vacuum in the tank. After closing the purge valve, pressure increase is measured by a tank pressure sensor. If vacuum increases above a defined threshold a leak is detected. As part of the vacuum check, purge flow is verified. For vapor generation test and EVAP system diagnosis, see Figure-Figure .

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.

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 1.
- 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.
- 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 1: EVAP System - Monitoring Structure Flow Chart (Test Group VBM2.8VJGKEK)
Courtesy of BMW OF NORTH AMERICA, INC.