Fuel Evaporative Emission Control (EVAP) System
The EVAP system reduces HC emissions by storing and then routing evaporated fuel from fuel tank and carburetor's float chamber through charcoal canister to intake manifold for combustion in cylinders at the proper time.
With ignition off, hydrocarbons from carburetor float chamber pass through de-energized outer vent control valve and into charcoal canister. Also, evaporated fuel from fuel tank passes and is stored in charcoal canister through a check valve on the canister.
With ignition switch in "ON" position and engine not running, outer vent control valve is energized, blocking movement of fuel vapor from carburetor's float chamber. Vapors from fuel tank can still move and be stored in canister.
With coolant temperature above 109°F (55°C) and engine running at 1600-1900 RPM, ECM energizes vacuum switching valve (VSV) which connects engine's manifold to canister purging it of stored vapors.
With engine running above 2290 RPM, throttle position switch closes, vacuum switching valve (VSV) remains energized and fuel vapors are purged from canister into intake manifold. If deceleration occurs, throttle position switch opens, ECM detects change and turns off VSV stopping canister purge to prevent an excessive amount of HC emissions.
When there is high pressure in fuel tank, canister valve No. 4 is open allowing HC from tank to be absorbed into canister. See Fig 1 and EVAP SYSTEM OPERATION chart.
When there is high vacuum in fuel tank, canister valve No. 4 closes, valve No. 5 opens and check valve in fuel filler cap opens. With these 2 valves open, air is pulled into tank to equalize pressure.