EVAP Evaporative Emissions
Evaporative emissions were installed on vehicles in the late 1960's when they realized that 10-20% of the vehicles emissions occurred while the vehicle was at rest. The basic system installed on these early model vehicles were vacuum controlled. The enhanced EVAP system found on today's vehicles is more complex and best diagnosed with a good visual inspection and a scanner.
These enhanced systems have a tank pressure sensor and a vent valve to inform the ECM of current conditions. In fact it's common for a vehicles check engine light to be on indicating a failure with the EVAP system when actually the customer left the gas cap loose. This is because the ECM is using this pressure sensor to monitor and sense the loss of tank pressure through the loose gas cap.
Charcoal Canister: The canister contains a charcoal insert that absorbs and retains HC's (hydrocarbon emissions) until they can be blended into the air fuel mixture in the intake manifold. Later model vehicles have the canister located close to the gas tank to better capture the 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 ECM only after certain conditions like vehicle and engine speed have been met. If this valve sticks in the open position fuel trim will be enriched.
Fuel Tank Pressure Sensor: This is an input used by the ECM 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 used by the ECM to help it monitor purge valve operation. The vent valve is used on enhanced OBD II EVAP systems.
