A diagnostic method and system includes a control valve configured to control an amount of air drawn into an evaporative emissions (EVAP) system through an air filter and a vapor canister, and a pressure sensor configured to measure pressure in the EVAP system. The system also includes a controller configured to detect an engine idle-to-off transition and, in response to detecting the engine idle-to-off transition: receive a first pressure from the pressure sensor, fully open a purge valve connected between the vapor canister and an intake port of an engine, fully close the control valve, monitor one or more second pressures received from the pressure sensor, and detect a malfunction of the EVAP system based on the first pressure, at least one of the one or more second pressures, and a diagnostic threshold.