Corolla & Tercel
Fuel evaporation system is controlled by a thermo switch (TS), vacuum switching valve (VSV) and a speed and temperature-controlled computer. When coolant temperatures are low, no vacuum is present at the VSV. When engine warms up, TS signals computer and vacuum is present at VSV.
When vehicle is stopped or running at low speed, computer interprets signals from the speed sensor and acts on the VSV, closing passage to intake manifold. This routes fuel vapors into the charcoal canister to be stored.
When vehicle reaches 1600 RPM on Corolla and Tercel, computer signals the VSV to open. This allows accumulated fuel vapors from the charcoal canister and fuel tank to be drawn into the intake manifold and burned in the combustion chamber.
A check valve in the charcoal canister is used to balance fuel tank pressure and prevent tank collapse. The outer vent control valve allows float bowl vapors to the canister when ignition is "OFF".