|LANCE’S CORNER – AUTO REPAIR HELP|
EVAPORATIVE CONTROL SYSTEM (EVAP)
Fuel vapors from the EVAP canister can only be drawn into the engine under certain conditions. If canister purge occurs, under the wrong conditions, engine performance and emissions can be adversely affected. The canister purge valve on most engines, is electronically activated by the engine control module to provide purge only under specific parameters. Canister purge is normally only commanded on when the vehicle is moving, the engine coolant temperature is at a specified level and the throttle is open. Before the arrival of computerized emission controls, canister purge was controlled using a thermal vacuum switch and a ported vacuum. The thermal vacuum switch is designed to only supply vacuum when a specified temperature is reached. Ported vacuum is a vacuum source that is only available when the throttle is opened. Vehicle speed was not a consideration in enabling canister purge. The fuel EVAP system was purged anytime the throttle was opened and when the engine was warm.
The fuel tank is sealed using a special fuel tank cap. The fuel tank cap is designed to provide an airtight seal, unless high levels of pressure or vacuum are present in the fuel tank. If this occurs, fuel vapors are released into the atmosphere through a pressure relief valve contained in the fuel tank cap.
(Lance owned his own auto repair shop for 30 years before retiring in 2006.)