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Evaporative Emission Control (EVAP) System

The evaporative emission control (EVAP) system captures gasoline fumes and other emissions produced when fuel evaporates within the gas tank or fuel system. The EVAP system then returns these vapors to the combustion process to keep harmful chemicals from reaching the air when the vehicle is not running.

Common EVAP system problems

The most obvious symptom of an EVAP system problem is the Check Engine light. A common (and easy-to-fix) cause of this is a loose gas cap. If you've recently filled up on fuel, check that the gas cap is fully tightened.

If the Check Engine light remains on after checking the gas cap, there may be a bigger problem in the EVAP system, including a system leak or failed component. Your car should be safe to drive, but may be causing a lot of excess pollution, and should be inspected by a professional mechanic. If you have a tool to check for diagnostic codes, you can also look up your OBD-II code for more information.

EVAP system components


Fuel tank

The fuel tank stores the fuel. Inside the tank, there is a float that signals the computer systems and/or fuel gauge as to how much fuel is in the tank. On newer vehicles, the fuel pump, fuel, filter, and fuel pressure regulator may also be housed inside the fuel tank.

EVAP system components: canister purge valve

The canister purge valve allows stored fuel vapors in the containment canister to be recycled back into the engine combustion process when necessary.

EVAP system components: charcoal canister

In order to prevent fuel vapors from escaping into the atmosphere, a brick of charcoal inside the charcoal canister collects the vapors and later purges them so they can burn in the combustion process.

EVAP system components: canister vent valve

The canister vent valve closes the fuel tank vent to prevent outside air from entering the EVAP system during an evaporative system leak test.

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