Evaporative Emissions System: Description and Operation

Evaporative Emission Control System (EVAP):

The evaporative emission control system traps fuel vapors emitted from the fuel in the fuel tank and stores them in a charcoal canister until the engine is running and the coolant temperature exceeds 55°C (131°F).
When the engine coolant temperature is below 55°C (131°F) the solenoid valve (No. 1 purge control) is off, no vacuum is applied to the No. 1 purge control valve, and no gas fumes are pulled into the engine.
When the engine coolant temperature reaches 55°C (131°F) and is idleing, the solenoid valve (No. 1 purge control) is off, no vacuum is applied to the No. 1 purge control valve, and no gas fumes are pulled into the engine. This is to prevent an overrich air/fuel ratio during idle.
When the engine coolant temperature reaches 55°C (131°F) and is operating above idle, the solenoid valve (No. 1 purge control) is turned on, vacuum is applied to the No. 1 purge control valve, and gas fumes pass through the orifice (A) and into the engine where they are burned.
When the engine coolant temperature reaches 70°C (158°F) and the engine speed is above 1500 RPM the solenoid valve (No. 2 purge control) comes on and vacuum is applied to the vacuum switch which allows fumes to bypass the orifice and increase evaporative gas flow to the engine.
The components of the evaporative emission control system are the: distributor, air flow meter, coolant temperature sensor, atmospheric pressure sensor, idle switch, inhibitor switch, clutch switch, neutral switch, charcoal canister, separator, check and cut valve, vacuum valve switch, solenoid valve (No. 2 purge control valve, solenoid valve (No. 1 purge control valve), and the engine control unit.

CHARCOAL CANISTER

The charcoal canister stores gasoline vapors when the engine is not running.

FUEL VAPOR SEPARATOR

The fuel vapor separator prevents fuel from flowing into the charcoal canister.

CHECK AND CUT VALVE

The check and cut valve controls the pressure in the fuel tank.

VACUUM SWITCH VALVE

The vacuum switch valve opens or closes additional purge line.

SOLENOID VALVE (NO. 2 PURGE CONTROL)

The No. 2 purge control solenoid valve controls the vacuum to the vacuum switch valve.

SOLENOID VALVE (NO. 1 PURGE CONTROL)

The No. 1 purge control solenoid valve controls line to No. 1 purge control valve on the charcoal canister.

ENGINE CONTROL UNIT

The engine control unit detects signals from input sensors and switches and controls the purge control solenoid valves.