Here's how the evaporative emissions and fuel system work in your car.

The EVAP system is responsible for releasing harmful gasoline fumes from the gas tank into the atmosphere, creating smog.

It consists of the gas tank, filler neck, cap, vacuum lines, switching valves and the charcoal canister.

When the tank is refilled, the fill control valve opens to allow the construction fumes in the tank to escape to the charcoal canister. The charcoal absorbs the vapor and releases it through the purge valve into the engine so it can burn.

When the tank is full, the filling check valve has a float that rises and closes the vent line via a diaphragm. This prevents liquid fuel from entering the charcoal canister.

The vacuum switching valve allows the ECU to pull and maintain a vacuum on the evaporative system for self-diagnosis. Using the vapor pressure sensor, it can determine whether there are leaks and illuminate an indicator light.

This self-diagnosis is part of every car with OBD2 and is essential for passing an emissions test.

The fuel tank has also been cut open, revealing the bulkheads inside, as well as the high and low points of the tank and the intake check valve.

Vacuum lines between the gas tank, charcoal canister and air filter ensure air is routed for proper venting, purging and self-diagnosis.

Caution must be exercised when working on the fuel system as gasoline fumes are extremely flammable.

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