Evaporative fumes from the fuel tank are vented through hoses into a canister containing activated carbon. The activated carbon absorbs and holds fuel vapors when the engine is not operating. When the engine is started and engine speed is greater than idle (purge at idle would cause too rich a mixture), engine vacuum draws fuel vapors from the canister into the engine. Regulation of vapors through this purge line is controlled by a PCM-controlled canister purge control solenoid.
Carbon canisters are either open or closed design. When the engine is started on open canister models, engine vacuum draws outside air into canister either through the top or through a filter in bottom of canister. This helps to purge vapors from the activated carbon.
Note. Models without fuel tank pressure control valves may use a special pressure/vacuum relief fuel tank filler cap or other external relief device.