Circuit Description

Evaporative (EVAP) emission large leak test applies vacuum to EVAP system and monitors vacuum decay. The Powertrain Control Module (PCM) monitors Fuel Tank Pressure (FTP) sensor signal to determine vacuum decay rate. At an appropriate time, the PCM turns EVAP canister purge valve on (open) and EVAP canister vent valve on (closed). This allows engine to draw a vacuum on EVAP system. At a calibrated time, or vacuum level, the PCM turns purge valve off (closed), sealing the system, and monitors FTP sensor input to determine EVAP system vacuum. If system is unable to achieve calibrated vacuum level, or if vacuum level decreases too rapidly, DTC P0440 will set.