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 uncomplicated and vacuum controlled. Enhanced EVAP systems found on today's vehicles are computerized and typically diagnosed with a visual inspection, a multimeter, 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. Late-model vehicles have an enhanced evaporative system equipped with a canister located close to the tank to capture fuel vapor during refueling. The ECM typically purges the canister in gear after the engine has warmed and reached a specific temperature.
Purge Solenoid: The purge solenoid is responsible for venting the tank's fuel vapor into the intake manifold. The ECM purges the system only after meeting certain conditions like engine temperature and speed. A faulty purge solenoid can result in a rough idle and difficulty starting.
Fuel Tank Pressure Sensor: This is an input used by the ECM to indicate if the system is leaking into the atmosphere. This input, along with other sensor inputs, determines if there is a fault in the tank's seal.
Vent Valve: The vent allows fresh air to enter the canister. The ECM also uses it to monitor purge valve operation. OBD II Enhanced EVAP systems contain a vent valve.