Theory Of Operation

Fig 1: EVAP System Component Identification & Location

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, pressure in the system will decrease as well and will eventually turn into a vacuum if no leaks are present. Even the smallest of leaks can be accurately detected in this manner. The ESIM has multiple functions. There are two weighted seals that keep the system normally closed from the atmosphere. The weighted seals are used to maintain the system pressure between +1 inch of water and -2 inches of water. Anytime (engine-on or engine-off) that pressure or vacuum reaches these thresholds, the weights will lift and provide relief. There is also a vacuum actuated switch that closes when the vacuum reaches a calibrated value. This is beneficial because the induced vacuum during a subsequent declining temperature will achieve the switch closed (pass threshold) sooner than if the tank had to decay from a higher built up pressure.

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 in the Fuel Tank by drawing in air from the atmosphere from the Intake Manifold. 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. 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 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 of 1051 minutes 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 - 4200 minutes). When the monitor records a valid switch closure (small leak passing event) the fail timers are reset. Separate timers are running in the background to verify that this monitor is completing on a regular basis.

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.

Shortly after the vehicle has achieved closed loop fueling during the next drive cycle, and when the enable conditions are met, the Purge Solenoid is opened to create a 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 inch of water within a calibrated time threshold, it is determined to be a general evaporative system failure (P0440)  .
• If vacuum is created and the switch has closed, the PCM stops purge 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 purge and monitors the switch closure time. If the switch stays closed longer than a maximum calibrated time before opening, it is determined that a large leak is not present and the Small Leak Monitor will continue to run until the accumulative monitor increments. If no ESIM Switch closures were recorded during the entire accumulated timers  , 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 of 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 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 in a further controlled manner allowing a more accurate test result. If the PCM detects a failure during the intrusive test, a purge system performance fault is set (P0441)  .