Abstract: A process and apparatus are disclosed for estimating evaporative fuel emission losses from a vehicle having a hydrocarbon-fueled engine operating under control of a microprocessor-based powertrain controller, a fuel tank with an evaporated fuel emission control system comprising a fuel vapor adsorbtion means connected to the tank and engine, and a diagnostic system that detects malfunctions in the vapor adsorbtion means. The process and apparatus measures temperature inside the vehicle passenger compartment at successive times when the engine is not running and determines the lowest temperature during partial diurnal and diurnal cooling cycles and uses such temperature along with any malfunction data to estimate evaporated fuel loss during a test period.