What does it mean?
The volatility of gasoline has both its pros and cons. Liquid gasoline cannot burn, but the ease at which it evaporates makes it’s vapors useful in gasoline-powered internal combustion engines around the world. On the other hand, since gasoline boils as low as 95 F, and evaporates at even lower temperatures, basically until it freezes, at some -50 F, depending on the exact blend.
Because gasoline is composed of a number of compounds harmful to plant and animal life, not to mention the environment, gasoline containers needs to be sealed to prevent vapors from escaping. The problem is that a perfectly sealed gasoline container, such as the gas tank in your car, wouldn’t work very well. Depending on construction, the fuel pump could eventually deform, collapse, or break a sealed gas tank.
Refueling would be difficult as well, because air in the tank would have a difficult time escaping at the same time as new gas is being introduced via the fuel filler tube. To prevent the aforementioned problems, the evaporative emissions control system, typically referred to as the EVAP system, keeps these things under control.
Using a system of valves and an activated-charcoal canister, the EVAP system allows for air to enter, as fuel is being pumped out, as well as for air to exit, as fuel is being pumped in. During refueling, the charcoal canister captures any vapors in the escaping air. Then, while the engine is running, the purge valve allows the engine to burn off any vapors caught in the canister.
This way, harmful vaporized hydrocarbons and other chemicals are safely burned, the only result being water and carbon dioxide. As with all systems, the EVAP system, controlled by the ECM (engine control module), includes a number of self-checks to make sure that it is indeed performing properly, reducing evaporative emissions from making it to the atmosphere.
In case of a fault, the ECM will set an evaporative emissions diagnostic trouble code (DTC) and illuminate the MIL (malfunction indicator lamp). One common problem in the EVAP system occurs when the ECM detects a vacuum when there shouldn’t be one, for example, if the purge valve is stuck open. In this case, DTC P0496 – EVAP System High Purge Flow will be set in system memory.