This SAE Recommended Practice describes a procedure for measuring evaporative emissions from fuel systems of passenger cars and light trucks. Emissions are measured during a sequence of laboratory tests that simulate typical vehicle usage in a metropolitan area during summer months: (1) A 1 hr soak representing on diurnal cycle in which temperature of fuel in the vehicle's tank is raised from 60 to 84 F. (2) A 7 mile run on a chassis dynamometer. (3) A 1 hr hot soak immediately following the 7 mile run. The method for measuring weight of fuel vapors emitted during the test employs activated carbon traps connected to the fuel system at locations where vapors from these openings are adsorbed by the traps@ and the gain in weight of the traps represents the fuel evaporative emissions. The test sequence and method for measuring emissions are applicable to vehicles either with or without systems or devices to control fuel evaporative emissions. Although they have been used successfully with a wide range of vehicles equipped with a variety of control devices@ they should not be applied indiscriminately to new or unique vehicles or fuel systems. For example@ based on experience that temperature excursions of the fuel tank in parked vehicles follow those of ambient air@ the test sequence prescribes heating of the fuel tank to simulate a diurnal soak. Any control system designed to alter the relation between fuel and ambient temperatures will not be properly evaluated in the test sequences prescribed.