Evaporative Emissions System: Description and Operation

As a part of the fuel system, vehicles are equipped with a fuel evaporative emission control system designed to meet federal and state requirements in effect at the time of production.

Evaporative emission systems have been designed and tested to exceed 193,080 km (120,000 miles) or 10 years of vehicle use. No maintenance or service should be required unless components have been damaged.

FILL CONTROL VENT SYSTEM
Fill limiting is accomplished through fill pipe configuration or external vent tubes within the fuel tank filler pipe and fuel tank. The vent system is designed to permit at least 10 percent fuel tank volume air space when the fuel tank is filled to capacity. This air space provides for thermal expansion of fuel, as well as being an aid to the in-tank vapor vent system.

This system provides a vapor space above the gasoline surface in the fuel tank. This area is sufficient to permit adequate breathing space for the evaporative emission valve under static and most dynamic conditions. This is accomplished by the evaporative emission valve that is centrally located on the upper surface of the fuel tank.

All evaporative emission valves make use of a small orifice in order to allow only vapor and not fuel to pass into the tube running forward to the evaporative emissions canister. On all vehicles, the assembly mounts directly to the top of the fuel tank using a fuel vapor separator seal.
Fuel vapors trapped in the sealed fuel tank are vented through the orificed evaporative emission valve in the top of the fuel tank. The vapors leave the evaporative emission valve through a single vapor tube and continue to the evaporative emissions canister for storage, until they are purged to the engine.

FUEL TANK EVAPORATIVE EMISSION SYSTEM
The evaporative emission system limits the amount of fuel vapor a vehicle can release to the atmosphere, enabling the vehicle to meet current federal and state requirements for fuel evaporation.

The following are components of the evaporative emission system:
- Sealed fuel tank
- Fuel tank filler cap
- Evaporative emission valve
- Evaporative emissions canister
- Evaporative emission canister valve (some engines)
- Evaporative emission canister purge valve (some engines)

The system works by capturing a very high percentage of fuel vapors that might otherwise escape from the fuel tank. It then directs these vapors to the evaporative emissions canister where they are stored while the engine is not running. When the engine is running, and conditions are acceptable, the vapors are purged out of the evaporative emissions canister and into the engine. This clears the evaporative emissions canister and allows it to accept more fuel vapors the next time the engine is shut off.

The system is basically the same for all vehicle and engine tubes: a hose or tube runs from the fuel vapor valve at the top of the fuel tank to the evaporative emissions canister to allow fuel vapors to pass from the tank to the evaporative emissions canister. An evaporative emission return tube runs from the evaporative emissions canister to the engine, allowing vapors to be purged from the evaporative emissions canister. Some engines use additional valves in one or more of these tubes to provide additional control over how much or when fuel vapor will be allowed to pass.

ENGINE EVAPORATIVE EMISSION SYSTEM

This system consists of:
- Evaporative emission canister valve (some vehicles)
- Evaporative emissions canister
- Solenoid vent valve and purge regulator valve (some vehicles)
- Secondary air bypass valve (some vehicles)
- Thermal vent valve (some vehicles)

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).

Use Ford-approved parts for replacement of fuel and vapor hoses and tubes. Original equipment components are designed to resist most environmental conditions encountered in the evaporative emission system.