Section 03-13: Evaporative Emissions | 1996 F-150, F-250, F-350, F-Super Duty and Bronco Workshop Manual |
Fuel Tank Evaporative Emission System
The evaporative emission system limits the amount of fuel vapor a vehicle may 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:
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 (6007) 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. A evaporative emission return tube (9G271) 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:
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:
Fill control venting.
Tank vapor venting and storage system.
Fuel vapor return system.
Fuel vapor purge system.
Pressure and vacuum relief system (fuel tank filler cap and tank vent valve).