Small Leak Test

The engine off natural vacuum, (EONV), diagnostic is the small leak detection diagnostic for the EVAP system. The EONV diagnostic monitors the EVAP system pressure or vacuum with the key OFF. The EONV utilizes the temperature changes and the resulting naturally occurring vacuum or pressure in the fuel tank immediately following a drive cycle. When the vehicle is driven, the temperature in the fuel tank rises. When the vehicle is parked with the engine OFF and key OFF, the temperature in the fuel tank will continue to rise for a period of time, and then begin to decrease. The EONV diagnostic relies on this temperature change, and the corresponding pressure change to determine if an EVAP system leak is present. The EONV diagnostic is designed to detect leaks as small as 0.51 mm (0.020 in). The diagnostic can determine if a small leak is present based on vacuum or pressure readings in the EVAP system. When the EVAP system is sealed a finite amount of pressure or vacuum will be observed. When a 0.51 mm (0.020 in) leak is present, little or no pressure or vacuum is observed. If the test reports a failing value, DTC P0442 will set.