How The Emissions Control Canister Works

The emissions control canister contains charcoal and is designed to prevent the gasoline vapors emitted by the gasoline in the fuel tank of automobiles from entering the atmosphere. The emissions control canister is mounted in a non-conspicuous, safe place, usually in one of the rear wheel wells. When the cars engine is not running the gasses radiating from the gasoline in the fuel tank are stored in the emissions control canister. The vapors are stored within this charcoal filled canister to be purged and burned when the running engine has reached certain predefined conditions. The emissions control canister in some automobiles will store any liquid fuel that has entered the canister in a reservoir designed into the bottom of the canister. The liquid fuel is stored in a separate reservoir to protect the integrity of the charcoal contained in the main emissions control canister housing.
There are three control methods used to purge the vapors from the emissions control canister, the first of which is controlled by the throttles position without a valve on the canister. A throttle body system may have a vacuum connecting the canister to a ported vacuum source designed as part of the throttle body. At any level higher than idle, the throttle may be positioned in will fresh air will be pushed into the emissions control canister. When the fresh air is added to the canister, it causes the fuel vapors to be purged into the throttle body through the port at a constant rate.
The second control method is by a vacuum valve that modulates the flow, also by throttle position. A diaphragm valve on the top of the canister opens when the vacuum ported from the throttle body opens it. When this valve is opened oxygen and fuel vapors are drawn into the intake manifold normally through the same port used to control the PCV system. The valve cycle under this control cycle is considered to be slaved to the throttle. The more you open the throttle the more the emissions control canister is purged.
The last of the three control methods is when the ECM activates a solenoid valve on the emissions control canister. This control method may be used only when the following pre-determined conditions are reached: The engine is within normal operating temperature, the engine has been running for a pre-determined amount of time, the vehicle speed has exceeded a predetermined rate and the throttle is open more than a predetermined level. As the solenoid is activated, full vacuum is applied from the manifold to the diaphragm of the purge valve, opening it completely. When the diaphragm fully opens the valve, fresh air rushes into the canister at a high rate, quickly purging the fuel vapors. When this happens, the ECM solenoid valve on the emissions control canister purges the canister.