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 get 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. As temperature increases, so does pressure inside the system. And conversely, as temperature decreases, vacuum is created if no leaks are present. The Evaporative System Integrity Monitor (ESIM) Switch has multiple functions. There are two weighted seals that keep the system normally closed from the atmosphere. The two weighted seals are used to regulate the system pressure between 250 Pa (+1" H2O) and -500 Pa (-2" H2O). There is also a vacuum actuated switch that closes when the vacuum reaches a calibrated value. During cooling, the internal weights seal the evaporative system from the atmosphere and allow the switch to close. If the switch closes it indicates that the system is not leaking. Only the ESIM Switch is used for leak check diagnostics. The Fuel Tank Pressure (FTP) Sensor is not used for diagnosing the system for small leak or large leak testing. 

ESIM SWITCH STUCK CLOSED MONITOR:  At ignition off, the state of the ESIM switch is evaluated. If the switch is open, a pass flag is set and PCM will complete power down. If the switch is closed, the PCM will wait a calibrated delay time and open the OBD Vent Valve  venting the vacuum in the Canister allowing the ESIM Switch to open. If the switch opens, a pass flag is set and the PCM will power down. If the PCM detects that the ESIM Switch is still closed after a calibrated time, an error is detected and a switch stuck closed failure is set at the next engine run cycle.

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 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 and stores the engine shut down time.

   • After a maximum 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 1.0" H2O 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 PCM stops purging and monitors the switch closure time. If the switch opens before a calibrated time, a large leak is present. Two consecutive failure events will mature a fault (P0455). 
• If vacuum is created and the switch has closed, the PCM stops purging and monitors the switch closure time. If the switch stays closed longer than a calibrated period of time, it is determined that a large leak is not present and the Small Leak Monitor will continue to run until the accumulative monitor reaches its threshold. If no ESIM Switch closures were recorded during the entire accumulated timer,  it is determined that a small leak is present (P0456). 

PURGE FLOW MONITOR:  The operation of the Purge Solenoid and evaporative purge flow is monitored using inputs from the Fuel Tank Pressure Sensor. The Purge Flow Monitor will only run if the previous engine off Small Leak event was a pass. 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.

• The non-intrusive purge monitor  runs during normal operation once the enable conditions are met and looks for a calibrated increase in vacuum in the fuel tank with increased purge flow, referred to as side 1. If side 1 passes, the purge monitor looks for a calibrated decrease in vacuum in the Fuel Tank with decreasing purge flow, referred to as side 2. If side 2 passes, the purge monitor is complete. If the purge flow monitor fails either side 1 or side 2, or does not complete both side 1 and side 2 within a specified time, an intrusive test is initiated to verify the results from the non-intrusive test.
• The intrusive diagnostic  increases and decreases the purge flow in a more controlled manner to allow a more accurate test result. If the PCM detects a failure during the intrusive test, a purge system performance fault is set (P0441). 

Only the Fuel Tank Pressure (FTP) Sensor is used for Purge Performance diagnostics. The ESIM Switch is not used for diagnosing the purge system.