Evaporative Emissions System: Description and Operation

Evaporative Emission Control System Schematic:

SYSTEM DESCRIPTION
Fuel vapors are collected from the fuel tank, through a calibrated pressure relief valve in the tank. Vapors are directed through vapor hoses to the charcoal canister, where they are condensed and stored. When the engine is started again and the vehicle driven, a solenoid valve opens allowing fuel vapors to be purged from the canister and burned in the engine along with the regular air/fuel charge.

Vapor Canister:

VAPOR CANISTER
This evaporative system uses a two tube canister, located in the left rear wheel well of the vehicle. The canister is filled with grains of charcoal to condense and store fuel vapors. A filtered vent in the bottom of the canister allows air into the canister to mix with the fuel vapors during purging.


EVAPORATIVE CONTROL
A normally open solenoid valve controls vacuum to the purge passage in the charcoal canister. Under cold or idle conditions, the solenoid is turned "ON" by the PCM, blocking vacuum to the canister purge passage. The solenoid valve is turned "OFF" to allow purging when all of the following conditions are present:

1. Engine is warm.
2. After the engine has been running a specified time.
3. Above a specified road speed.
4. Above a specified throttle opening.

IN-TANK PRESSURE CONTROL VALVE
The in-tank pressure control valve, a combination roll-over, integral pressure, and vacuum relief valve, is located with the fuel sending unit in the fuel tank. When vapor pressure in the tank exceeds 1" Hg (5 kPa) the valve opens and allows vapors to vent to the canister. When the tank pressure drops below the opening point of the valve, it will close. This allows vapors to be trapped in the fuel tank. The valve protects against vacuum build up in the fuel tank, and prevents liquid fuel from entering the canister during aggressive driving maneuvers.