Evaporative Emissions System: Description and Operation

Evaporative Emissions Control System Schematic:

The evaporative emission control system used on this vehicle is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon storage canister to retain the vapors when the vehicle is not operating. When the engine is running the fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.

Inverted Function Vapor Canister:

VAPOR CANISTER
Gasoline vapors from the fuel tank flow into the tube labeled "Fuel Tank". Any liquid fuel goes into a reservoir in the bottom of the canister to protect the carbon bed above. These vapors are absorbed into the carbon. The canister is purged when the engine is running above idle speed. Ambient air is allowed into the canister through the air tube in the top. The air mixes with the vapor and is drawn into the intake manifold.

Canister Control Valve:

CANISTER CONTROL VALVE
The canister control valve is located in the canister purge line, which is connected between the fuel tank and the canister. Manifold vacuum opens this valve to allow purge of both the fuel tank and canister at the same time. The purge line is connected to a ported vacuum source on the throttle body. Purge occurs only when the throttle is above a certain opening, thus preventing purge operation under conditions of low ported vacuum, such as deceleration.

Fuel Tank Pressure Control Valve:

TANK PRESSURE CONTROL VALVE
This valve is used to regulate vapors venting from the fuel tank to the canister. When fuel tank pressure exceeds the relief point (8.1 kPa), the valve opens and the vapors are allowed to the canister. The valve closes when tank pressure drops.