Reading an article just now..
Evaporative Emission Control (EVAP)
The EVAP system prevents fuel vapors in the fuel tank from escaping into the atmosphere. The EVAP system traps fuel vapors from the fuel tank and temporarily stores them in the charcoal canister, see the diagram. When the engine is running under certain conditions, the fuel vapors are purged from the canister and burned inside the engine. The purge valve precisely controls the amount of fuel vapor that is purged from the charcoal canister.
In modern cars, the purge valve is an electrically-operated solenoid, that is controlled by the engine computer. When the engine is off, the purge valve is closed. When the engine is running and fully warmed up, the engine computer gradually opens the purge valve to allow some amount of fuel vapor to be moved from the charcoal canister to be burned in the engine. The purge flow is monitored by a number of sensors. If the purge flow is less or more than is expected under the conditions, the computer illuminates the "Check Engine" light.
Purge valve problems
The most common problem with the purge valve is when it sticks or does not close fully. This may cause the "Check Engine" warning light to come on. In some cars, a stuck-open purge valve can cause difficulty starting right after refueling at a gas station: for the first few seconds the engine may run rough and stumble. Purge valve problems are common in many cars. In some early-2000's Hyundai models (Elantra, Santa Fe, Tucson, Tiburon), a stuck-open purge valve is fairly common to cause the "Check Engine" light*
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