The fuel vapor canister, also known as a charcoal canister because it is filled with charcoal, is part of a vehicle's pollution control system and is used to capture fuel vapors emanating from both the fuel tank and the carburetor. The vapors are pulled into the canister by a vacuum as the engine is started, and then fed back into the fuel system to be burned when the engine is running. It is a component of a vehicle's evaporative emission control (EVAP) system and prevents the evaporating fumes from reaching the atmosphere while also maintaining proper fuel tank pressures.
In the mid 1970s, automobile manufacturers began installing pollution control systems on engines to help reduce emissions put into the atmosphere from automobiles. Studies found that a great deal of the hydrocarbons (HCs) that were emitted from automobiles were due to evaporative gasses. By placing the fuel vapor canister in the fuel system, the gasses are drawn in and then burned instead of being expelled. This makes for a cleaner burning engine and a better environment for everyone. The system is also used on fuel-injected vehicles with the only difference being the gasses are sent into the intake rather than the carburetor.
There is no required service for the device beyond keeping the hoses and canister clean and free of debris. A cracked hose will cause the "check engine" light to come on in a vehicle's dashboard display area, alerting the driver to a potential problem with the vehicle. Other areas that may cause the warning light to be seen are a faulty fuel tank cap or a loose or incorrectly-tightened cap. There is also a valve on the canister and the intake that are both vacuum-activated and could cause the warning light to come on should a malfunction occur.
The most common reason for a failure in the fuel vapor canister is a crack in the canister or a hose. In order to test it, nitrogen is applied to create pressure, and then it is tested periodically for a drop in pressure, which will indicate a fault in either the canister or a hose. Once the problem component is identified, the car owner should repair or remove the affected piece. Nitrogen is always used to test for leaks because introducing air into the canister could lead to a dangerous fire, explosion, or other incident caused by adding oxygen to the fuel vapors.