Evaporative Emissions System: Description and Operation

Fuel Tank Evaporative Emission System

The evaporative emission system limits the amount of fuel vapor a vehicle releases to the atmosphere in accordance with current federal and state requirements for fuel evaporation.

The evaporative emission system consists of:

- Sealed fuel tank

- Pressure/vacuum relieved fuel tank filler cap

- Evaporative emission valve

- Evaporative emissions canister (EVAP canister)

- Evaporative emission canister purge valve (EVAP canister purge valves)

- Associated tubes, hoses, clamps, and fittings

All gasoline-fueled engines are equipped with a fuel evaporative emission system. Fuel vapors are vented through the evaporative emission valve mounted on top of the fuel tank to the carbon canister. A hose or tube directs vapors to the evaporative emissions canister and from the evaporative emissions canister to the engine permitting fuel vapor canister purging, and vapor consumption by the engine.

The 3.0L and 4.0L engines incorporate an evaporative emission canister purge valve in the tube/hose between the evaporative emissions canister and throttle body vacuum fitting. The evaporative emission canister purge valve is operated by the powertrain control module (PCM) and regulates the quantity and duration of vapor delivery to the throttle body .

The evaporative emission system functions by capturing a very high percentage of fuel vapors vented from the fuel tank. These vapors are directed to the evaporative emissions canister where they are stored when the engine is not running. When conditions are acceptable (engine running, manifold vacuum and temperature within specification), both stored and recently generated vapors are purged into the engine air intake stream for consumption by the engine. Thus, the evaporative emissions canister is purged of stored vapors and prepared for future fuel vapor storage.

Fuel vapor canister purging occurs as fresh air is introduced into the system, passed through the evaporative emissions canister, and into the engine air intake stream. Fresh air is drawn into the evaporative emissions canister through the canister air vent. The fresh air is directed to the evaporative emissions canister where it passes over the activated charcoal bed and releases stored fuel vapors. The fresh air/fuel vapor mixture is vented from the evaporative emissions canister to the throttle body vacuum fitting where it is introduced into the engine air intake stream. Thus, the evaporative emissions canister is purged and prepared for future fuel vapor storage.

Engine Evaporative Emission System

This system consists of:

- Evaporative emission canister valve (some vehicles)

- Evaporative emission canister purge valve (some vehicles)

- Purge regulator valve (some vehicles)

- Secondary air bypass valve (some vehicles)

- Thermal vent valve (some vehicles)

- Evaporative emissions canister

Not all of these components are used on any one system, since usage depends on the calibration of the complete vehicle.

System control and operation are accomplished through five separate basic functions in the system:

1. Fill control venting.

2. Tank vapor venting and storage system.

3. Fuel vapor return system.

4. Fuel vapor purge system.

5. Pressure and vacuum relief system (fuel tank filler cap and tank vent valve).