Evaporative Emission (EVAP) Leak Check Monitor

EVAPORATIVE EMISSION (EVAP) LEAK CHECK MONITOR

The EVAP leak check monitor is an on board strategy designed to detect a leak from an opening equal to or greater than 0.508 mm (0.020 inch) in the enhanced EVAP system. The correct function of the individual components of the enhanced EVAP system, as well as its ability to flow fuel vapor to the engine, is also examined. The EVAP leak check monitor relies on the individual components of the enhanced EVAP system to either allow a natural vacuum to occur in the fuel tank or apply engine vacuum to the fuel tank and then seal the entire enhanced EVAP system from the atmosphere. The fuel tank pressure is then monitored to determine the total vacuum lost (bleed-up) for a calibrated period of time. Inputs from the engine coolant temperature (ECT) sensor or cylinder head temperature (CHT) sensor (if equipped), intake air temperature (IAT) sensor, mass airflow (MAF) sensor, vehicle speed, fuel level input (FLI) and fuel tank pressure (FTP) sensor (if equipped), are required to enable the EVAP leak check monitor.

During the EVAP leak check monitor repair verification drive cycle, clearing the continuous diagnostic trouble codes (DTCs) and resetting the emission monitors information in the powertrain control module (PCM), bypasses the minimum soak time required to complete the monitor. The EVAP leak check monitor does not run if the ignition is turned OFF after clearing the continuous DTCs and resetting the emission monitors information in the PCM. The EVAP leak check monitor does not run if a MAF sensor concern is present. The EVAP leak check monitor does not initiate until the heated oxygen sensor (HO2S) monitor is complete.

If the vapor generation is high on some vehicle enhanced EVAP systems, where the monitor does not pass, the result is treated as a no test. Therefore, the test is complete for the day.

Some vehicle applications have an engine OFF natural vacuum (EONV) check as part of the EVAP leak check monitor.

Engine On EVAP Leak Check Monitor - Fiesta
The engine on EVAP leak check monitor is executed by the individual components of the enhanced EVAP system as follows:

1. The PCM uses inputs from the engine coolant temperature (ECT) sensor, the fuel level input (FLI), the intake air temperature (IAT) sensor, the mass airflow (MAF) sensor, the NVLD ambient air temperature sensor, the vehicle speed sensor (VSS) and the NVLD module to determine conditions of the enhanced EVAP system. The combination of these signals are used by the PCM to determine when to activate the EVAP leak check monitors.
2. The fuel tank pressure is inferred by the PCM based on a message from the NVLD module and other engine parameters. The NVLD module message is based on the position of the NVLD vacuum switch and the NVLD ambient air temperature sensor during calibrated conditions of the EVAP system.
3. The EVAP canister purge valve creates a vacuum in the fuel tank for the large leak check. In order to detect if the canister purge valve will open and allow the vacuum to be released, a canister purge valve check is initiated after the NVLD vacuum switch is detected as closed.
4. The NVLD module uses the NVLD vacuum switch to seal the EVAP system from the atmosphere. The NVLD vacuum switch is closed when a vacuum is created in the fuel tank. The NVLD vacuum switch position is monitored for correct operation after the NVLD vacuum switch is detected as closed. Correct operation of the NVLD vacuum switch is determined by opening the canister purge valve with the engine OFF to relieve the vacuum in the fuel tank and force the NVLD vacuum switch to open. If the NVLD vacuum switch does not open within a calibrated amount of time, a mechanical switch error is detected.
- If the NVLD vacuum switch is closed at the beginning of the large leak test, then the EVAP system does not have a large leak and the test is passed.
- If the NVLD vacuum switch is open at the beginning of the large leak test, the PCM opens the EVAP canister purge valve to a calibrated amount creating a vacuum in the fuel tank. The NVLD vacuum switch position is monitored by the NVLD module. If the NVLD vacuum switch closes within a calibrated period of time after the EVAP canister purge valve is open, the system does not have a large leak and the test is passed.

5. On a normally operating EVAP system, a vacuum is generated inside the fuel tank as the temperature of the fuel decreases.







Engine On EVAP Leak Check Monitor - All Others
The engine on EVAP leak check monitor is executed by the individual components of the enhanced EVAP system as follows:

1. The EVAP canister purge valve controls the flow of vacuum from the engine and creates a target vacuum on the fuel tank.
2. The EVAP canister vent solenoid seals the EVAP system from the atmosphere. It is closed by the PCM (100% duty cycle) to allow the EVAP canister purge valve to achieve the target vacuum on the fuel tank.
3. The FTP sensor is used by the engine on EVAP leak check monitor to determine if the target vacuum necessary to carry out the leak check on the fuel tank is reached. Some vehicle applications with the engine on EVAP leak check monitor use a remote inline FTP sensor. Once the target vacuum on the fuel tank is achieved, the change in fuel tank vacuum over a calibrated period of time determines if a leak exists.
4. If the initial target vacuum cannot be reached, DTC P0455 (gross leak detected) sets. The engine on EVAP leak check monitor aborts and does not continue with the leak check portion of the test.

For some vehicle applications, if the initial target vacuum cannot be reached after a refueling event and the purge vapor flow is excessive, DTC P0457 (fuel cap off) sets.

If the initial target vacuum is exceeded, a system flow concern exists and DTC P1450 (unable to bleed-up fuel tank vacuum) is set. The engine on EVAP leak check monitor aborts and does not continue with the leak check portion of the test.

If the vacuum increase is quicker than expected, a blocked fuel vapor tube is suspected and if confirmed after an intrusive test, DTC P144A sets.

If the target vacuum is achieved on the fuel tank, the change in the fuel tank vacuum (bleed-up) is calculated for a calibrated period of time. The calculated change in fuel tank vacuum is compared to a calibrated threshold for a leak from an opening of 1.016 mm (0.040 inch) in the enhanced EVAP system. If the calculated bleed-up is less than the calibrated threshold, the enhanced EVAP system passes. If the calibrated bleed-up exceeds the calibrated threshold, the test aborts. The test can be repeated up to 3 times.

If the bleed-up threshold is still being exceeded after 3 tests, a vapor generation test is carried out before DTC P0442 (small leak detected) sets. This is accomplished by returning the enhanced EVAP system to atmospheric pressure by closing the EVAP canister purge valve and opening the EVAP canister vent solenoid. Once the FTP sensor observes the fuel tank is at atmospheric pressure, the EVAP canister vent solenoid closes and seals the enhanced EVAP system.

The fuel tank pressure build-up over a calibrated period of time is compared to a calibrated threshold for pressure build-up due to vapor generation.

If the fuel tank pressure build-up exceeds the threshold, the leak test results are invalid due to vapor generation. The engine on EVAP leak check monitor attempts to repeat the test again.

If the fuel tank pressure build-up does not exceed the threshold, the leak test results are valid and DTC P0442 sets.

5. If the 1.016 mm (0.040 inch) test passes, the test time is extended to allow the 0.508 mm (0.020 inch) test to run.

The calculated change in fuel vacuum over the extended time is compared to a calibrated threshold for a leak from a 0.508 mm (0.020 inch) opening.

If the calculated bleed-up exceeds the calibrated threshold, the vapor generation test is run. If the vapor generation test passes (no vapor generation), an internal flag sets in the PCM to run a 0.508 mm (0.020 inch) test at idle (vehicle stopped).

On the next start following a long engine OFF period, the enhanced EVAP system is sealed and evacuated for the first 10 minutes of operation.

If the appropriate conditions are met, a 0.508 mm (0.020 inch) leak check is conducted at idle. If the test at idle fails, DTC P0456 sets. There is no vapor generation test with the idle test.

6. The malfunction indicator lamp (MIL) is activated for DTCs P0442, P0455, P0456, P0457, and P1450 after 2 occurrences of the same concern and for DTC P144A after a sufficient number of completions. The MIL can also be activated for any enhanced EVAP system component DTCs in the same manner. The enhanced EVAP system component DTCs P0443, P0446, P0452, P0453, and P1451 are tested as part of the comprehensive component monitor (CCM).







Engine Off Natural Vacuum (EONV) EVAP Leak Check Monitor
The EONV EVAP leak check monitor is executed during ignition OFF, after the engine on EVAP leak check monitor is completed. The EONV EVAP leak check monitor determines a leak is present when the naturally occurring change in fuel tank pressure or vacuum does not exceed a calibrated limit during a calibrated amount of time. A separate, low power consuming, microprocessor in the PCM manages the EONV leak check. The engine OFF EVAP leak check monitor is executed by the individual components of the enhanced EVAP system as follows:

1. The EVAP canister purge valve is normally closed at ignition OFF.
2. The normally open EVAP canister vent solenoid remains open for a calibrated amount of time to allow the fuel tank pressure to stabilize with the atmosphere. During this time period the FTP sensor is monitored for an increase in pressure. If pressure remains below a calibrated limit the EVAP canister vent solenoid is closed by the PCM (100% duty cycle) and seals the EVAP system from the atmosphere.
3. The FTP sensor is used by the EONV EVAP leak check monitor to determine if the target pressure or vacuum necessary to complete the EONV EVAP leak check monitor on the fuel tank is reached. Some vehicle applications with the EONV EVAP leak check monitor use a remote inline FTP sensor. If the target pressure or vacuum on the fuel tank is achieved within the calibrated amount of time, the test is complete.
4. The EONV EVAP leak check monitor uses the naturally occurring change in fuel tank pressure as a means to detect a leak in the EVAP system. At ignition OFF, a target pressure and vacuum is determined by the PCM. These target values are based on the fuel level and the ambient temperature at ignition OFF. As the fuel tank temperature increases, the pressure in the tank increases and as the temperature decreases a vacuum develops. If a leak is present in the EVAP system the fuel tank pressure or vacuum does not exceed the target value during the testing time period. The EONV EVAP leak check monitor begins at ignition OFF.

After ignition OFF the normally open EVAP canister vent solenoid remains open for a calibrated amount of time to allow the fuel tank pressure to stabilize with the atmosphere. During this time period the FTP sensor is monitored for an increase in pressure. If pressure remains below a calibrated limit the EVAP canister vent solenoid is closed by the PCM (100% duty cycle) and seals the EVAP system from the atmosphere.

If the pressure on the fuel tank decreases after the EVAP system is sealed, the EONV EVAP leak check monitor begins to monitor the fuel tank pressure. When the target vacuum is exceeded within the calibrated amount of time the test completes and the fuel tank pressure and time since ignition OFF information is stored. If the target vacuum is not reached in the calibrated amount of time, a leak is suspected and the fuel tank pressure and time since ignition OFF information is stored.

If the pressure on the fuel tank increases after the EVAP system is sealed, but does not exceed the target pressure within a calibrated amount of time, the EVAP canister vent solenoid is opened to allow the fuel tank pressure to again stabilize with the atmosphere. After a calibrated amount of time the EVAP canister vent solenoid is closed by the PCM and seals the EVAP system. When the fuel tank pressure exceeds either the target pressure or vacuum within the calibrated amount of time, the test completes and the fuel tank pressure and time since ignition OFF information is stored. If the target pressure or vacuum is not reached in the calibrated amount of time, a leak is suspected and the fuel tank pressure and time since ignition OFF information is stored.

On ISO 14229 vehicles, a fast initial response occurs during the first 4 tests after the battery is disconnected or the DTCs are cleared. The PCM processes unfiltered data to quickly indicate a fault is present. The MIL illuminates if the PCM suspects a leak within 2 consecutive trips after a DTC clear or a battery disconnect using the fast initial response logic.

A step change logic becomes active after the 4th EONV monitor test. The step change logic detects an abrupt change from a no leak condition to a suspected leak condition. The MIL illuminates if the PCM suspects a leak within 2 consecutive trips using the step change logic.

During the EONV monitor test the PCM uses an exponentially weighted moving average to filter test data. The PCM uses this average after the fourth EONV test and illuminates the MIL on the first trip when the exponentially weighted moving average is greater than a calibrated threshold.

When a leak is suspected, DTC P0456 sets and the MIL is illuminated.

On non-ISO 14229 vehicles, when a leak is suspected, the PCM uses the stored fuel tank pressure and time since ignition OFF information from an average run of 4 tests to suspect a leak. Some vehicles use an alternative method of a single run of 5 tests to determine the presence of a leak. If a leak is still suspected after 2 consecutive runs of 4 tests, (8 total tests) or one run of 5 tests, DTC P0456 sets and the MIL is illuminated.

5. The EONV EVAP leak check monitor is controlled by a separate low power consuming microprocessor inside the PCM. The fuel level indicator, fuel tank pressure, and battery voltage are inputs to the microprocessor. The microprocessor outputs are the EVAP canister vent solenoid and the stored test information. If the separate microprocessor is unable to control the EVAP canister vent solenoid or communicate with other processors, DTC P260F sets.
6. The MIL is activated for DTCs P0456 and P260F. The MIL can also be activated for any enhanced EVAP system component DTCs in the same manner. The enhanced EVAP system component DTCs P0443, P0446, P0452, P0453, and P1451 are tested as part of the CCM.







Natural Vacuum Leak Detection (NVLD) Small Leak Monitor
The engine off NVLD small leak monitor is executed by the individual components internal to the NVLD module as follows:

1. The PCM uses inputs from the ECT sensor, the IAT sensor, the MAF sensor, the VSS and the NVLD module to determine conditions of the enhanced EVAP system. The combination of these signals are used by the PCM to determine when to activate the EVAP leak check monitors.
2. When the ignition is turned OFF and the calibrated conditions are met the PCM sends a message to the NVLD module to begin the engine off NVLD monitor.
3. The EVAP canister purge valve is normally closed with the ignition OFF.
4. The NVLD leak check monitor uses the naturally occurring change in fuel tank pressure as a means to detect a leak in the EVAP system.

The small leak check monitor is controlled by a separate low power consuming microprocessor inside the NVLD module.

The small leak check monitor is executed with the ignition OFF, after the engine running EVAP leak check monitor is completed. The heat generated while the engine is running warms the fuel in the fuel tank. When the engine is turned OFF a natural vacuum is generated by the fuel cooling in the fuel tank. On a normally operating EVAP system this vacuum closes the NVLD vacuum switch. The NVLD vacuum switch position is checked after 10 minutes from engine shut down. If the NVLD vacuum switch is closed the small leak monitor passes.

If, after 10 minutes from engine shut down the NVLD vacuum switch is not closed, and the NVLD ambient air temperature sensor change is more than 8°C (14°F) over the next 24 hour period without the vacuum switch closing, the test fails.

The PCM receives a message from the NVLD module at ignition ON and then engine running, indicating the EVAP system has either passed or failed the small leak test.

A vacuum decay test is executed as a rationality test to the NVLD small leak check monitor. The vacuum decay rate is determined by the calculated fuel tank pressure, leak size, the fuel tank fill level, the NVLD ambient temperature sensor, and the fuel type. Tank pressure is determined by tank fill level, AAT sensor, the EVAP canister purge valve opening, and NVLD vacuum switch position prior to engine OFF. If either the NVLD small leak check monitor passes or the vacuum decay test passes the PCM considers the EVAP system passed the leak test. If the NVLD small leak check monitor fails and the vacuum decay test passes the PCM considers the EVAP system passed the leak test

5. On a normally operating EVAP system, a vacuum is generated inside the fuel tank as the temperature of the fuel decreases.