Enhanced Evaporative Emission Systems
Evaporative emissions were installed on vehicles in the late 1960s when they realized that 10-20% of the vehicle's hydrocarbon emissions occurred while the vehicle sat at rest. The basic systems installed on these early model vehicles were vacuum controlled. Enhanced EVAP systems found on today's vehicles are computerized and more complex. They diagnosed with proper visual inspection and a scan tool.
Charcoal Canister: The canister contains a charcoal insert that absorbs and retains fuel vapor until it's purged into the intake manifold for combustion. The PCM typically purges the canister after the engine has warmed and reached a specific temperature. Late-model enhanced evaporative equipped vehicles have the canister located close to the tank to capture fuel vapor better as the vehicle is being fueled.
Purge Solenoid: The purge solenoid is responsible for venting the tank's fuel vapor into the intake manifold. The PCM purges the system only after certain conditions like engine temperature and speed have been met. A faulty purge solenoid can result in a rough idle, and difficulty starting.
Fuel Tank Pressure Sensor: This is an input used by the PCM to indicate if the system is leaking to the atmosphere. This input, along with other sensor inputs, determines if there is a fault in the tanks 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 found on enhanced OBD II EVAP systems.