Theory Of Operation

Courtesy of CHRYSLER GROUP, LLC

EVAPORATIVE SYSTEM OVERVIEW:  The Powertrain Control Module (PCM) monitors the Evaporative Emission System operation. The two main areas being monitored are the integrity of the system against leaks and the ability of the system to purge fuel vapor from the canister to the Intake Manifold. The basic strategy used is that in a sealed system  , pressure will naturally increase or decrease in relation to temperature. The Evaporative System Integrity Monitor (ESIM) Switch is used for all leak fault detection.  There is a vacuum actuated switch that closes when the vacuum reaches a calibrated threshold. If the switch closes it indicates that the system is not leaking.

ESIM SWITCH STUCK CLOSED MONITOR:  At ignition off, the state of the ESIM Switch is evaluated. If the switch is open, a pass flag will set so the PCM power down process can complete. If the switch is closed, the PCM will wait a calibrated delay time and open the Purge Solenoid. In a normally functioning system, this will relieve the vacuum trapped in the Charcoal Canister allowing the switch to open. When the switch opens, a pass flag is set and the PCM will power down. If the ESIM switch does not open, after a calibrated time, an error is detected and a switch stuck closed failure event is set (P0452)  . Two consecutive failed events will mature a fault.

SMALL LEAK MONITOR:  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 2-5 minutes.
   NOTE:

   The Engine on timer is clipped to a maximum of 26 minutes on any given trip.

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 and stores the engine shut down time.

   • After a maximum time of 17.5 hours without an ESIM Switch closure during the event.

This monitor will increment  the accumulation fail timers  until both  have reached a calibrated threshold (Engine on - 100 minutes and Engine off - 70 hours). When the monitor records a valid switch closure (small leak passing event)  the fail timers are reset.

LEAK SIZE DETERMINATION:  If the PCM did not see an ESIM Switch closed signal during the previous ignition off cycle, the event was valid, and there is a cold start, an intrusive leak test is run to determine if a large leak is present. Immediately after start-up, while the engine is cold the Purge Solenoid is opened to create vacuum in the evaporative system to a calibrated vacuum point that is beyond the ESIM Switch closing threshold. The pass/fail time will vary based on the total fuel volume at the time of the test. 

• If the switch does not close at all during purging, because of a switch that is stuck open or vacuum cannot be created below -250 Pa within a calibrated time, it is determined to be a general evaporative system failure (P0440)  .
• If vacuum is created and the switch has closed, the Purge Solenoid is commanded off and the PCM monitors the switch closure time. If the switch opens before a calibrated threshold, a large leak is present. Two consecutive failure events will mature a fault (P0455). 
• If vacuum is created and the switch has closed, the Purge Solenoid is commanded off and the PCM monitors the switch closure time. If the switch stays closed longer than a calibrated threshold, it is determined that a large leak is not present. If the Large Leak diagnostic does not fail before the accumulated small leak timer reaches it's threshold it is determined that a small leak is present (P0456).