Theory Of Operation

Fig 1: EVAP System Components

Courtesy of CHRYSLER GROUP, LLC

EVAPORATIVE SYSTEM OVERVIEW: 

The Powertrain Control Module (PCM) monitors the Evaporative Emission System operation. The primary concern being monitored is the integrity of the system against leaks. The operation of the Purge Solenoid and evaporative purge flow is also monitored. This is accomplished using inputs from the ESIM Switch and Fuel Tank Pressure Sensor.

PURGE FLOW MONITOR: 

The Purge Flow Monitor will only run if the Small Leak Monitor recorded a pass on the previous ignition off event and the test data was valid. Because the leak detection diagnostics can only verify that the fuel tank system is sealed while the purge valve is closed, it cannot determine if the purge line between the solenoid and Intake Manifold is pinched or leaking. The Purge Flow Monitor is needed to verify these failure modes. The Purge Flow Monitor works on the premise that as flow through the system increases, so does the pressure drop in the system. The PCM monitors the Fuel Tank Pressure Sensor and looks for increasing vacuum in the Fuel Tank with increasing purge flow. Conversely, it looks for decreasing vacuum in the Fuel Tank with decreasing flow.

• With the engine running and enable conditions met, the non-intrusive purge monitor  looks for a calibrated increase in vacuum in the fuel tank with increased purge flow, referred to as phase 1. If phase 1 passes, the purge monitor looks for a calibrated decrease in vacuum in the Fuel Tank with decreasing purge flow, referred to as phase 2. If phase 2 passes, the purge monitor is complete. If the purge flow monitor fails either phase, or does not complete both phases within a specified time, an intrusive test is initiated to verify the results from the non-intrusive test.
• The intrusive diagnostic  uses the same two phases to analyze the system. However, the intrusive test actuates the Purge Solenoid such that the differences are much more pronounced allowing a more accurate test result. If the PCM detects a failure during the intrusive test, a purge system performance fault is set (P0441).

VALID SMALL LEAK MONITOR EVENT:  This is an accumulative monitor and the data from each valid event is recorded and added to the previously recorded events. The PCM timer records the engine on/drive cycle and engine off time for each small leak monitor event. For an event to be valid the PCM must see; 

1. An engine on/drive cycle for a minimum of 10 minutes.
   NOTE:

   The engine on timer will stop counting after a maximum of 26 minutes.

2. And, when the engine is shut down, an engine off timer starts. There is a 12 minute delay time in which the PCM will ignore ESIM Switch input. The engine off timer period will continue to count until one of the three conditions exist:
   • The engine is started without a switch closure during the event.
   NOTE:

   At the next key on cycle a determination is made as to whether the event was valid and the information is kept.

   • An ESIM Switch closed input is received after the 12 minute delay during the event.
   NOTE:

   If the switch closed input is received, the PCM records that the switch has closed, stores the engine shut down time, then goes to sleep.

   • After a maximum of 1051 minutes without an ESIM Switch closure during the event.