See Figure 1
The heart of this system is a charcoal canister located in the engine compartment. Fuel vapor that collects in the carburetor float bowl or gas tank and which would ordinarily be released to the atmosphere is stored in the canister because of the affinity of the charcoal for it.
In order to restore the ability of the charcoal to hold fuel, fresh air is drawn through the charcoal under certain operating conditions, thus drawing the fuel back out and burning it in the combustion chambers.
At idle speed, or when the engine is cold, the addition of any fuel vapor to the correct mixture would cause excessive emissions. A port in the carburetor or fuel injection system throttle body allows the fuel to be drawn out of the canister only after the throttle has been opened past the normal idle position (the port is located above the position of the throttle at idle). If there is no vacuum, the canister purge valve remains closed. The flow of air and fuel are prevented when the engine is cold via a thermal valve. This valve prevents the vacuum signal going to the canister purge valve from passing through it until the engine reaches a certain temperature. On turbocharged vehicles, two vacuum signals are sent to the purge valve so that the system will work only above idle speed and below full throttle.
When the canister purge valve opens, air is drawn under very slight vacuum from the air intake hose located between the air cleaner and carburetor or injection system.
REMOVAL & INSTALLATION
The canister or canisters should be replaced at specified intervals, as shown on the maintenance chart. Make sure all hoses are clamped and not dry rotted or broken. The canister filter, if equipped, should be inspected, cleaned or replaced at least every two years. Any clogging of the filter will inhibit air flow through the canister. Two different types of valves were used in the canister lines--refer to Emission Controls for a detailed description of their function and necessary servicing. To replace the canister:
- Remove the two connecting hoses from the canister.
- Loosen and remove the canister retaining band bolt.
- Remove the canister.
- Check the hoses, replace any that are cracked, soft or collapsed. Install the new canister and connect the hoses.