Monitoring Process

The canister close valve and the EVAP canister purge control valve are closed and an increase in fuel tank pressure caused by fuel evaporation occurs (compensation gradient). Afterwards, the canister close valve and the purge control valve are opened. The canister close valve will be shut if the purge control valve reached its final duty cycle.

Fig 1: Illustrating EVAP System Processes

Courtesy of KIA MOTORS AMERICA, INC.

1. Gross leak detection

   A gross leak is assumed if negative pressure is built-up below a certain threshold. In this case, the EVAP canister purge control valve is closed and the canister close valve is opened. Then, the diagnosis is completed.

2. Small leak detection

   The EVAP canister purge control valve is closed as soon as the maximum vacuum level is achieved. The rate of vacuum loss will be corrected by the above compensation gradient. A small leak is assumed if this corrected rate of vacuum loss is above a certain threshold. Then, the canister close valve is opened and the diagnosis is completed.