The function of EVAP is to retain fuel vapors so that they do not escape into the atmosphere. Once the vapors are captured by the EVAP, they are sent through a carbon canister, through vacuum lines to the engine, where they are subsequently burned. It should be noted that these vapors do not enter the EVAP system during idling. It is only when the demand for fuel is high or when the EVAP system vapor pressure is above the set limit.
There is a sensor in the Evaporative Emission Control System that is also called EVAP, which converts system pressure changes into signal voltages which are sent to the PCM to indicate that fuel adjustments should be made to ensure the optimum air/fuel mixture at all times.
When there are changes in exhaust gas composition, some varying signal voltages are transmitted to the PCM, which are used by the PCM to reduce the fuel vapor flow or close the bleed valve to maintain or restore the air/fuel ratio as needed.