If that's about six inches in diameter, that's the charcoal canister. Fumes that vaporize mainly in the fuel tank on warm days travel to that canister and are stored there. When the engine is running, those fumes are drawn out and burned in the engine.
About the only thing that ever goes wrong with them is if they are cracked in a crash or the vacuum hoses become deteriorated and crack or fall off. Leaks in those hoses are monitored very closely on '96 and newer models. While those leaks are fairly rare, at the worst if a hose is leaking you might smell fuel if you stick your nose close to it.
Wednesday, June 1st, 2011 AT 5:28 AM