More than likely it has to do with the charcoal canister which is part of the EGR (exhaust gas recirculation). The unburned fuel in the exhaust is routed to the canister where it is stored until it reaches a certain level. At that time a solenoid causes the gases to be purged from the canister and returned to the gas tank. This is supposed to be a way of making the vehicle more fuel effecient,
