Diagnostic Overview - EVAP Emissions - PHEV

Courtesy of CHRYSLER GROUP, LLC

The Evaporative Non-Integrated Leak Monitor (ENILM) diagnostics are performed in the engine off (after run) state.  The diagnostic will run after the ignition is off for 6 hours. The reason for the wait time is that there are cases where the Fuel Tank pressure increases greatly after ignition off (due to high temperatures) and is too large for the vacuum pump in the Evaporative Leak Check Module (ELCM) to make the pressure negative. This could cause a false failure during small leak testing. After the calibrated wait time the Powertrain Control Module (PCM) will initiate the Evaporative System small leak diagnostic. The ELCM and the Fuel Tank Isolation Valve (FTIV) are key components used in diagnosing the system. Refer to the Description and Operation of the ELCM and the FTIV for more specific details on how the components function for diagnostics. Refer to EMISSIONS CONTROL / EVAPORATIVE EMISSIONS / DESCRIPTION AND OPERATION .

There are a series of phases to the small leak diagnostics. The figures below are a flow chart and graph of the entire diagnostic test timing. Each individual phase of the diagnostic is described in detail below the figures.

Courtesy of CHRYSLER GROUP, LLC

Courtesy of CHRYSLER GROUP, LLC

Phase 0: Before any components are commanded on, the Fuel Tank side of the system is evaluated for a leak and the Fuel Tank Pressure (FTP) Sensor reading rationality is checked.  The FTP Sensor signal is monitored for one minute. If the pressure reading is within the calibrated minimum and maximum thresholds, relatively steady, and not equal to atmospheric pressure, then the PCM assumes that the Fuel Tank side of the system is sealed and the FTP Sensor reading is valid. The Fuel Tank Evaluation Counter will increment one count. This counter will factor in to the direction of the diagnostic following phase 3. After the counter reaches a calibrated threshold, the diagnostic will continue on to phase 5, open the FTIV and test the entire system for a leak, regardless of the results in phases 0 - 3. This is done as a confirmation that the Fuel Tank side of the system is indeed sealed and the FTP Sensor is accurate.

Phase 1: The diagnostic will check the 0.020" Reference Orifice for too much or too little flow, a Change-Over Valve that is stuck open, and an ELCM Vacuum Pump that is stuck off.  During phase 1 the PCM takes an initial atmospheric pressure (BARO) reading from the ELCM Pressure Sensor. The Change-Over Valve inside the ELCM remains closed (the Change-Over Valve is vented to atmosphere at this time), isolating the ELCM from the rest of the Evaporative System, and the ELCM Vacuum Pump is turned on. During this time the ELCM is drawing outside air through an internal 0.020" orifice only. This is done to determine a baseline pass/fail criteria for the small leak check.

• If the pressure reading is changing, but still above 15.0 kPa (2.18 psi) after 5.0 seconds has elapsed, the PCM determines that the normally closed Change-Over Valve is stuck open (P24C0).
• After the vacuum is allowed to stabilize for approximately 10.0 seconds, the pressure reading through the 0.020" Reference Orifice is recorded. The reading is expected to be within a calibrated upper and lower range. Too much vacuum (negative pressure) indicates an orifice that is restricted and sets (P24B9). Too little vacuum indicates excessive flow through the reference orifice and sets (P24B9). If the pressure reading remains near atmospheric pressure, between approximately 70 and 100 kPa (10.0 and 14.5 psi) without any change, the PCM determines that the ELCM Vacuum Pump is stuck off (P1406).

Phase 2: The ELCM Pressure Sensor rationality is checked.  After the previous diagnostic passes, the baseline 0.020" reference pressure reading from the ELCM Pressure Sensor is compared to a known good threshold and adjusted for the current atmospheric pressure (BARO) reading. Review the table below for an example of the calibrated pressure limits based on BARO. The values in the chart showing the upper and lower limit specifications are approximate values used for informational value only.

Readings Taken	psi (kpa)	psi (kpa)	psi (kpa)	psi (kpa)
Atmospheric Pressure (BARO)	10.15 (70)	11.6 (80)	13.05 (90)	14.5 (100)
0.020" Reference Pressure Upper Limit	-0.585 (-4.04)	-0.599 (-4.13)	-0.61 (-4.21)	-0.619 (-4.27)
0.020" Reference Pressure Lower Limit	-0.14 (-0.95)	-0.15 (-1.04)	-0.165 (-1.14)	-0.18 (-1.24)

• If the pressure sensor signal is outside of the calibrated threshold, the sensor is considered irrational (P24B9).

Phase 3: The diagnostic checks the fresh air side of the system for a leak, the Change-Over Valve for a stuck off (closed) condition and the FTIV for a stuck open condition. 

• Immediately after the Change-Over Valve is energized (opened) at the beginning of phase 3, the pressure should rise back near BARO and then start drawing back into a vacuum. If the system pressure does not rise above a calibrated threshold within 2.0 seconds, the Change-Over is determined to be stuck closed (P24C1).
• As vacuum is being created and the pressure is dropping, it is compared to the Fuel Tank Pressure Sensor reading on the Fuel Tank side of the system. If the difference between the fresh air side pressure and the Fuel Tank side pressure is not more than a calibrated threshold after a calibrated period of time, the FTIV is determined to be stuck open (P2450).
• As the pressure stabilizes in the fresh air side, the PCM makes a determination if the system is leaking by comparing the ELCM Pressure Sensor reading to the 0.020" small leak threshold obtained during the first 0.020" Reference Orifice check. If the system vacuum created goes below the 0.020" small leak threshold, it is determined that there is no leak in the fresh air side of the system. If the system is determined to be leaking, the PCM determines the size of the leak (small leak or large leak) based on how far above the threshold the system stabilizes at. If the system vacuum (negative pressure) is equal to the small leak threshold, the fresh air side is determined to have a small leak (P04ED). If the vacuum stabilizes above the small leak threshold, then a large leak is detected (P04EF). The leak check will run for a maximum time of 180 seconds.

Phase 4: A determination is made of the system. 

• If the fuel level is less than 95% and ANY  of the following are true, the diagnostic will continue on to Phase 5  :
  • Tank evaluation counter threshold met (calibrated threshold is set between 4 - 6 counts)
  • Fuel Tank evaluation failed in Phase 0 (Fuel Tank pressure was unstable or equal to atmospheric pressure, indicating a possible leak, faulty FTP Sensor, or unstable tank pressures)
  • Phase 3 leak detection evaluation failed
• If the fuel level is more than 95%, or  the fuel level is less than 95% and ALL  of the following are true, the diagnostic will skip directly to Phase 7  :
  • Tank evaluation counter threshold has not been met
  • Fuel Tank evaluation passed in Phase 0
  • Phase 3 leak detection evaluation passed

Phase 5: The diagnostic checks the Fuel Tank side of the system for a leak and the FTIV for a stuck closed condition. 

• When the FTIV is energized (opened), and the Change-Over Valve is de-energized (closed), the FTP Sensor reading should return back to atmospheric pressure. If the FTP Sensor reading does not return to atmospheric pressure within a calibrated period of time, the FTIV is determined to be stuck closed (P2451).
• The Change-Over Valve is then re-energized (opened) and the entire system is now evacuated and monitored for a leak. During this phase the leak check can run for a maximum time of 1850 seconds. Once the system pressure stabilizes, the pressure reading is again compared to the 0.020" small leak threshold obtained during the first 0.020" Reference Orifice check. If the system vacuum (negative pressure) is equal to the small leak threshold, the Fuel Tank side is determined to have a small leak (P04EE). If the vacuum stabilizes above the small leak threshold, then a large leak is detected (P04EC). If the system vacuum created goes below the 0.020" small leak threshold, it is determined that there is no leak in the Fuel Tank side of the system.

If the leak test fails  in Phase 5, the diagnostic continues to Phase 6. If all tests pass  in Phase 5, the diagnostic skips to Phase 7.

Phase 6: The FTIV is turned off (closed) and the Fuel Tank Pressure is monitored for vapor generation.  This is performed because, as mentioned at the beginning, excessive system pressure that is too large for the ELCM Vacuum Pump to create vacuum can cause a false failure of the system.

• If the system builds pressure quickly or builds excessive pressure, then the previous failure is not  recorded and the test is complete.
• If the pressure reading remains stable for a calibrated period, then the leak test results are considered valid  and the diagnostic will continue to Phase 7.

Phase 7: The diagnostic validates the leak check, checks for the Purge Solenoid stuck open and the ELCM Vacuum Pump stuck on.  The Purge Solenoid is opened shortly to vent system vacuum through the engine and then closed again. The PCM also performs a second BARO check with the Purge Solenoid open. Shortly after the Purge Solenoid closes, the Change-Over Valve is turned off (closed) and a second 0.020" Reference Orifice check is performed and compared to the first check for validation.

• If the system is not able to pull a vacuum after the Purge Solenoid is closed, the Purge Solenoid is determined to be stuck open (P04DF).
• If a vacuum is created through the 0.020" Reference Orifice and the value matches the initial reading, the test is valid and logs a pass/fail depending on the results of the leak test.
• If the second BARO check value varies from the first BARO check value by more than a calibrated amount, the pressure sensor rationality fails (P24B9) and the test is not valid.
• If the second orifice check value varies from the first orifice check value by more than a calibrated amount, the pressure sensor rationality fails (P24B9) and the test is not valid.
• After all testing has completed and passed, the ELCM Vacuum Pump is turned off. With the pump off, and the Change-Over Valve is closed, the ELCM Pressure Sensor reading should return to atmospheric pressure. If the pressure does not return to atmosphere, the ELCM Vacuum Pump is determined to be stuck on (P1405).