Evaporative Emissions System: Description and Operation

Purge Control System:

The Evaporative Emission Control System is designed to prevent fuel vapors from being emitted into the atmosphere from the fuel tank. The components incorporated into this system are: a Two-Way Check Valve, a Pressure-Vacuum Filler Cap, a Charcoal Canister, and connecting lines and hoses. Canister purging is controlled by the ECU through a Purge Control Solenoid Valve.
The Pressure-Vacuum Filler Cap is designed to hold a slight pressure on the fuel which occurs from normal heat expansion. The evaporation rate decreases under higher pressures, so the cap aids to reduce evaporative emissions. When vacuum in the tank exceeds specifications, the vacuum relief valve will allow atmospheric pressure to enter the tank. The 2-way check valve vents excess pressure and fuel vapors from the tank into the charcoal canister, when the pressure becomes positive and reaches its specified value. The Two-Way Check Valve is mounted on or near the fuel tank in the fuel vapor line to the canister.
The charcoal canister absorbs and stores fuel vapors until the ECU grounds the purge control solenoid. The purge control solenoid is spring loaded closed (blocks purge), and opens when grounded by the ECU. Manifold vacuum is then allowed to purge the canister based on a timer circuit. The ECU will ground the purge control solenoid when the coolant sensor indicates higher than 149°F and the Air Flow Sensor indicates an intake air flow rate of 2000 rpm (100 Hz). Once these parameters have been met, the ECU will ground the purge control solenoid for 3 1/2 minutes of ON time and then remove the solenoid ground for 25 seconds of OFF time. This will continue as long as the above parameters are met. During engine warm-up, the purge ON time is progressive. For example, after the coolant sensor indicates above 149°F, the first purge cycle may only be for one minute of purge, then 25 seconds of no purge. When the purge control solenoid valve opens, the fuel vapors in the canister are drawn into the intake manifold through purge lines. At the same time, fresh air is drawn through the filter at the bottom of the canister, purging the charcoal of fuel vapors.