The emissions control canister will release the stored fuel vapors from within it when the emissions control pressure high is reached. At the pre-determined emissions control pressure high one of three control methods become activated for the emissions control canister to purge the vapors from itself. The three control methods to purge the emissions control canister that may be used to purge the emissions control canister when the emissions control pressure high is reached are:
1. The position of the throttle on a throttle body system will determine when the emissions control canister is purged after the emissions control pressure high is reached. 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, that the throttle may be positioned in 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.
2. The second control method used when the emissions control pressure high is reached is by a vacuum valve that modulates the flow, also by throttle position. A diaphragm valve on the top of the canister is opened by the vacuum from the throttle body. 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.
3. The last of the three control methods to purge the canister when the emissions control pressure high is reached 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 its pre-determined normal operating temperature, the engine has been running for a pre-determined amount of time, the vehicle speed has exceeded a pre-determined rate and the throttle is open more than a pre-determined 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.