Theory Of Operation

WARNING: This page is about a different car, the 2009 Dodge Caliber. However, it is still accessible from the selected car via links, so may be relevant.

The EVAP Purge Monitor tests the integrity of the hoses/tube between the throttle body/intake and the fuel tank. The monitor is a two stage test and runs only after the EVAP system passes the small leak test. Stage one is non-intrusive. The Powertrain Control Module (PCM) monitors the purge vapor ratio and the Evaporative System Integrity Monitor (ESIM) Switch closed ratio. If the purge vapor ratio is above a calculated value, the monitor passes. If the ESIM switch closed ratio is greater than calculated value when purge flow is greater than a minimum value, the monitor passes. Stage two is an intrusive test and runs only if stage one does not pass. The PCM commands the purge solenoid to flow at a specified rate to force the purge vapor ratio to update. The ratio is compared to a calibrated specification. If it is less than specified, a one trip failure is recorded. This test can detect if the purge hose is off, obstructed, or the purge valve is not operational.