The vapor canister is an emission device which draws in vapors from evaporation and sends them to the fuel tank. To preserve the vapors so when the engine needs more fuel, the canister will release the stored vapor.
Fuel evaporator canisterThe canister was developed to trap the fuel vapors and send them to the engine. But the vehicles before that had vented fuel tank caps and a carburetor, but it releases the vapors to the atmosphere and would be the cause of smog. Now the engine generally comes with pressure relief fuel caps that to keep the tank from collapsing.
The carbon canisterThe carbon canister contains charcoal which is able to absorb the fuel vapor at large quantities. It is usually accompanied by a filter to keep out any dirt that flows in when the engine is drawn in by the vacuum.
Common issues with the vapor canisterA bad vapor canister can lead to a decline in fuel efficiency, leaks, and rough idling.