Evaporative Emissions System: Description and Operation

EVAPORATIVE EMISSION CONTROL SYSTEM
The evaporative emission control system prevents the escape of fuel vapors to the atmosphere under hot soak and engine OFF conditions by storing vapors in an Evaporative Emissions (EVAP) canister. The fuel vapor is stored in the EVAP canister until it is drawn into the intake manifold and burned when the engine is started. The system is controlled by the Powertrain Control Module (PCM), which operates the evaporative emission canister purge valve based on input signals.

FILL CONTROL/VAPOR VENT SYSTEM
Fill control is accomplished through the fuel tank filler pipe and a vent line, attached to the fuel tank filler pipe, which extends inside the fuel tank. The vent system is designed to provide an air space above the fuel level in a full fuel tank. The air space allows for thermal expansion of fuel and provides clearance between the fuel level, vapor orifice, and evaporative emission valve, which is mounted in the top panel of the fuel tank. The clearance is sufficient to allow vapor venting through the orifice under all static and most dynamic vehicle conditions.

THE FUEL TANK EVAPORATIVE EMISSION SYSTEM CONSISTS OF:
- Fuel tank (9002) and fuel tank filler pipe
- Pressure/vacuum relief fuel tank filler cap
- Evaporative emission tube shut off valve
- Vapor tube and hoses

THE ENGINE EVAPORATIVE EMISSION SYSTEM CONSISTS OF:
- Evaporative emissions canister
- Evaporative emission canister purge valve
- Evaporative emission front hose

System control and operation are accomplished through five separate basic functions in the system:
1. Fill control vent system.
2. Tank vapor venting and storage system.
3. Evaporative emission return system.
4. Evaporative emission purge system.
5. Pressure and vacuum relief system (fuel tank filler cap and tank vent valve).