Here's how the Evaporative emissions system work on a modern automobile.

The purpose of the EVAP system is to control the release of gasoline vapors entering the atmosphere, causing smog. It is a self-governing, mandated system that vents most harmful vapors into an external charcoal canister for refinement.
The system consists of several components: The gasoline tank, the fuel sending unit, temperature sensor, pressure sensor, vent and purge solenoids, charcoal canister, vacuum hoses, filler neck, gas cap and fuel check valve inside the tank.

When gasoline is being filled into a tank, vapors are routed to the charcoal canister for refinement. Air and water vapor are allowed to vent to atmosphere, allowing the tank to fill up seamlessly without trapping air.
During normal vehicle operation, gasoline vapors are routed to the charcoal canister and then sent to the air intake through a vacuum hose. The engine then reburns this vapor, with the ECU compensating accordingly for a richer mixture. The release is controlled by the purge valve.

A pressure sensor situated on the purge line (near the charcoal canister) allows for self regulation of the EVAP system through OBD2 protocols. During certain drive cycles, the ECU can detect if there is any leaks in the EVAP system and throw a check engine light accordingly.
It does this by drawing a vacuum on the system from the air intake, then sensing if there is any pressure drops (and the extent of the drop) due to a leaking system.

In this video, the entire EVAP system is gutted from an Infiniti G35, cut open, cross-examined and explained. The gas tank is cut open to examine the vapour cut off and liquid recovery floats, the dual-tank like system (due to RWD configuration) and the valving inside the filler neck.
Furthermore, the EVAP canister and vent valve is cut open to examine its role in the system.

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