EVAP System Description
The EVAP controls minimize the amount of fuel vapor escaping into the atmosphere. Vapor from the fuel tank is temporarily stored in the EVAP canister until it can be purged from the canister into the engine and burned.
The EVAP canister is purged by drawing fresh air through it and into a port on the intake manifold.
The purging vacuum is controlled by the EVAP canister purge valves.
Operation
When fuel tank pressure increases because of heat and fuel vapor generation, evaporated fuel vapor is sent to the EVAP canister.
When the engine coolant is hot enough, the PCM opens the evaporative emission (EVAP) canister purge valves to draw the stored fuel vapor in the EVAP canister into the intake manifold to be burned.
To regulate the amount of fuel vapor drawn into the engine, the PCM varies the EVAP canister purge valves opening using duty cycle control.
The PCM commands one purge valve open, once the duty cycle is at 100 %, the second purge valve will be commanded open to increase the flow of the fuel vapor.
When the outside air or other factor cools the fuel tank inside, the tank pressure decreases.
The EVAP canister vent shut valve is normally open and allows outside air to enter the fuel tank through the EVAP canister.
If there is a system malfunction or blockage in the vent hose, a pressure valve in the fuel fill cap opens and allows outside air to enter preventing system damage.
The PCM uses the FTP sensor and the EVAP canister vent shut valve to check for leaks in the system.
When the enabling conditions are met, the EVAP canister vent shut valve is closed, the PCM monitors for changes in the FTP sensor.