Evaporative Emissions System: Description and Operation

Evaporative Emission Control System (EVAP):

When the engine is not running, the EVAP system collects fuel vapors that would otherwise be discharged into the atmosphere and stores them in a charcoal canister. When the engine is started, vapors are directed to the intake and burned in the engine. System helps prevent raw hydrocarbon emissions and promotes fuel economy by eliminating fuel loss through evaporation.

The EVAP system is ECU controlled. System consists of a vapor separator, purge control solenoid valve, charcoal canister, two-way check valve, three-way check valve, vapor separator, ECU, and engine sensors.

ECU monitors operating conditions and regulates amount of evaporative gases drawn into the engine by alternately energizing and de-energizing the solenoid valve (duty cycle signal - "ON" time vs. "OFF" time). System operates when the following conditions are met.

^ Engine at normal operating temperature.
^ Driving in gear.
^ Accelerator depressed (idle switch off).
^ ECU in "closed loop" operation (oxygen sensor functioning normally).

CHARCOAL CANISTER
Canister contains activated charcoal to condense and store gasoline vapors from the fuel tank when the engine is not running.

CHECK AND CUT VALVE
Valve has three functions:

^ To prevent tank damage, vents fuel tank to atmosphere if tank pressure exceeds approx. 0.78 - 1.0 psi (5.37 - 6.87 kPa).
^ To prevent tank damage, allows atmospheric air to enter tank if tank pressure is approx. 0.14 - 0.71 psi (0.98 - 4.91 kPa) less than atmospheric.
^ Blocks fuel from leaking through EVAP system in event vehicle is overturned.

FUEL VAPOR SEPARATOR
Separates fuel vapors from liquid fuel to prevent saturation of the charcoal canister. Liquid fuel returns to the fuel tank from the bottom of the separator. A hose connected at the top of the separator collects vapors and routes them to the canister through the check and cut valve and two-way check valve.

TWO-WAY CHECK VALVE
Valve prevents fuel vapors stored in charcoal canister from returning to tank.

PURGE CONTROL SOLENOID VALVE
Valve is a normally closed, electrically operated vacuum solenoid valve. It gets battery power from the main relay when the ignition is turned "ON" and is energized when the ECU completes the circuit to ground. The solenoid operates on a duty cycle ("ON" time vs "OFF" time). Valve is connected in-line between PCV valve and charcoal canister. It vents the charcoal canister to the intake manifold when operating conditions are suitable.