Enhanced Evaporative Emission Systems
Evaporative emissions were installed on vehicles in the late 1960’s when they realized that 10-20% of the vehicle’s hydrocarbon emissions occurred while the vehicle was at rest. The basic systems installed on these early model vehicles were vacuum controlled. The enhanced EVAP system's found on today’s vehicles are computerized and more complex. They are best diagnosed with a good visual inspection and a scan tool.
Charcoal Canister: The canister contains a charcoal insert that absorbs and retains fuel vapor until it can be purged into the intake manifold for combustion. The PCM typically purges the canister after the engine has warmed and reached a certain temperature. Late model enhanced evaporative equipped vehicles have the canister located close to the tank to better capture fuel vapor as the vehicle is being refueled.
Purge Valve: The purge valve is responsible for venting the tanks fumes into the intake manifold. This is done by the PCM only after certain conditions like engine temperature and speed have been met. If this valve sticks in the open position, the fuel ratio will be enriched.
Fuel Tank Pressure Sensor: This is an input used by the PCM to indicate if the system is leaking to the atmosphere. This along with other sensor inputs determine if there is a fault in the systems seal.
Vent Valve: The vent allows fresh air to enter the canister. The valve is also used by the PCM to monitor purge valve operation. The vent valve is used on enhanced OBD II EVAP systems.