Evaporative Emissions System: Operation

When the engine is stopped, vapors formed in the gas tank or carburetor fuel bowl pass through a vent line to the charcoal canister(s). These canisters are filled with activated charcoal to collect and hold fuel vapors. When the engine is running, intake manifold vacuum draws the fuel vapors from the canister into the engine, where they are burned.

The fuel tank modifications prevent overfilling, and the sealed cap keeps expanding fuel from escaping out the cap. An expansion tank is located in the fuel tank on some models and externally on others. It allows the gasoline to expand without overflowing. A vapor separator prevents liquid fuel from reaching the charcoal canister.

An anti-run on valve is used on carbureted vehicles to control canister purging and engine dieseling. When the ignition is turned "OFF", the valve closes and cuts off purge air to the canister. It also connects the float chamber to manifold vacuum, which prevents fuel from being drawn through the carburetor into the engine and causing run on. When oil pressure drops to zero, the valve will be deactivated.