To reduce hydrocarbon (HC) emissions, evaporated fuel from the fuel tank is routed through the charcoal canister to the intake manifold for combustion in the engine. A fuel filler cap with a check valve allows air to enter the fuel tank as the fuel is used to prevent a vacuum build up and equalize the pressure, but does not allow fuel vapors to escape to the atmosphere. When the engine coolant temperature reaches 129°F (54°C), the vacuum switching valve opens to allow vapors trapped in the charcoal canister to enter the intake manifold, where they are drawn into the combustion chambers and burned with the fuel charge.
The EVAP system should be checked for proper operation every 60,000 miles. Check all hose and connections for leaks or deterioration and remove the charcoal canister to inspect for cracks or damage. Check for a clogged filter or frozen check valve by blowing low pressure compressed air into the tank and check that air flows freely to the canister port. Blow into the charcoal canister purge pipe and make sure no air flows from the tank connection or from the fresh air inlet at the bottom of the canister. If it does, replace the canister. Clean the pipe while holding the purge connection closed. Do not attempt to wash the canister in any way.
Check the Bi-metal Vacuum Switching Valve (BVSV) operation by allowing the engine to cool completely and blowing through the valve. When cold, the valve should be closed (no air should pass). Start and warm up the engine, then blow through the valve again. Air should now pass freely (switch open). Any results other than these, replace the BVSV.
The thermo switch is tested by allowing the engine to cool completely and checking for continuity across the thermo switch terminals. Start and warm up the engine and check that the thermo switch is open (no continuity) at normal operating temperature. If not, replace the thermo switch.