Engine Off Natural Vacuum (EONV) EVAP Leak Check Monitor
The EONV EVAP leak check monitor is executed during key 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 will manage the EONV leak check. The engine off EVAP leak check monitor is executed by the individual components of the enhanced EVAP system as follows:
- The EVAP canister purge valve, also known as the vapor management valve (VMV), is normally closed at key off.
- The normally open canister vent (CV) 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 CV is closed by the PCM (100% duty cycle) and seals the EVAP system from the atmosphere.
- 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 in-line FTP sensor. If the target pressure or vacuum on the fuel tank is achieved within the calibrated amount of time the test is complete.
- 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 key 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 key off. As the fuel tank temperature increases, the pressure in the tank will increase and as the temperature decreases a vacuum will develop. If a leak is present in the EVAP system the fuel tank pressure or vacuum will not exceed the target value during the testing time period. The EONV EVAP leak check monitor begins at key off.
After key off the normally open canister vent (CV) 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 CV 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 key 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 key 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 CV is opened to allow the fuel tank pressure to again stabilize with the atmosphere. After a calibrated amount of time the CV 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 key 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 key off information is stored.
When a leak is suspected, the PCM will use the stored fuel tank pressure and time since key off information from an average run of 4 tests to suspect a leak. If a leak is still suspected after 2 consecutive runs of 4 tests, (8 total tests) DTC P0456 is set and the MIL is illuminated.
- 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 CV solenoid and the stored test information. If the separate microprocessor is unable to control the CV solenoid or communicate with other processors DTC P260F is set.
- 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.