Chevrolet Trailblazer Service Manual page 213

Fuel pumped from an underground storage tank is cool but when exposed to a warmer
ambient temperature, will expand. Before controls were mandated, an owner might fill the
fuel tank with fuel from an underground storage tank and park the vehicle for some time in
warm area, such as a parking lot. As the fuel would warm, it would expand and should no
provisions or area be provided for the expansion, the fuel would spill out of the filler neck
and onto the ground, causing hydrocarbon (HC) pollution and creating a severe fire
hazard. To correct this condition, the vehicle manufacturers added overflow plumbing
and/or gasoline tanks with built in expansion areas or domes.
However, this did not control the fuel vapor emission from the fuel tank. It was determined
that most of the fuel evaporation occurs when the vehicle is stationary and the engine is
off. Most vehicles carry 5-25 gallons (19-95 liters) of gasoline. Should a large
concentration of vehicles be parked in one area, such as a large parking lot, excessive fuel
vapor emissions would take place, increasing as the temperature increases.
To prevent the vapor emission from escaping into the atmosphere, the fuel systems were
designed to trap the vapors while the vehicle is stationary, by sealing the system from the
atmosphere. A storage system is used to collect and hold the fuel vapors from the fuel
tank when the engine is not operating. When the engine is started, the storage system is
then purged of the fuel vapors, which are drawn into the engine and burned with the
air/fuel mixture.