The evaporative emission (EVAP) system is used to absorb fuel vapors from the fuel tank to reduce the amount of hydrocarbons emitted into the atmosphere. In a hot soak or refueling condition, fuel vapor pressure inside the fuel tank increases. In order to depressurize the fuel tank the excess air (which was drawn into the fuel tank as the fuel level decreased) and fuel vapor mixture in the fuel tank pass through the vacuum cut valve into the EVAP canister and out the EVAP canister vent valve. The EVAP canister contains activated carbon which collects the fuel vapors to prevent them from being emitted into the atmosphere with the escaping air. The fuel vapors are stored in the EVAP canister until they can be consumed by the engine during normal engine operation (i.e. not decel, idle, low engine coolant temperature, starting, and wide open throttle (WOT). During normal engine operation the powertrain control module (PCM) commands the EVAP canister purge valve ON which opens the EVAP canister purge valve. When the EVAP canister purge valve is open, intake manifold vacuum is applied to the EVAP canister which draws in fresh air and fuel vapors from the EVAP canister into the intake manifold. The PCM uses various sensor inputs to calculate the desired amount of EVAP purge flow. The PCM meters the purge flow by varying the duty cycle of the EVAP canister purge valve input signal. The evaporative emission (EVAP) system monitor is a self-test strategy within the powertrain control module (PCM) that tests the integrity of the EVAP system. When a fault occurs, the EVAP system monitor is reset to NO and a diagnostic trouble code (DTC) is set in the PCM memory.
After the DTC is repaired the vehicle drive cycle must be completed to reset the monitor in preparation for Inspection and Maintenance (I/M) testing. The PCM monitors the EVAP system for leaks, electronic EVAP components for irrationally high or low voltage levels sent to the PCM, and the EVAP system for proper operation. The EVAP system monitor uses the positive and the negative pressure leak test methods to test and activate the EVAP system. The positive pressure leak test uses fuel tank fuel vapor pressure (when the fuel tank temperature is sufficient) to test the system. During the positive pressure leak test the EVAP canister purge valve is closed (OFF), the EVAP canister vent valve is closed (ON), the manifold absolute pressure/barometric pressure (MAP/BARO) solenoid is ON (MAP/BARO sensor connected to barometric pressure) and the vacuum cut bypass valve is open (ON). The positive pressure test passes if the EVAP pressure sensor indicates a rise in EVAP pressure and the pressure holds until the EVAP canister vent valve is commanded open. The negative pressure leak test uses intake manifold vacuum to test the system. During the negative pressure leak test the EVAP canister purge valve is open (ON), the EVAP canister vent valve is closed (ON), and the MAP/BARO solenoid is OFF (MAP/BARO) sensor connected to manifold absolute pressure). The negative pressure test passes if the EVAP pressure sensor indicates a decrease in EVAP pressure equal to the pressure indicated from the MAP/BARO sensor and the pressure holds until the EVAP canister vent valve is commanded open.
After the DTC is repaired the vehicle drive cycle must be completed to reset the monitor in preparation for Inspection and Maintenance (I/M) testing. The PCM monitors the EVAP system for leaks, electronic EVAP components for irrationally high or low voltage levels sent to the PCM, and the EVAP system for proper operation. The EVAP system monitor uses the positive and the negative pressure leak test methods to test and activate the EVAP system. The positive pressure leak test uses fuel tank fuel vapor pressure (when the fuel tank temperature is sufficient) to test the system. During the positive pressure leak test the EVAP canister purge valve is closed (OFF), the EVAP canister vent valve is closed (ON), the manifold absolute pressure/barometric pressure (MAP/BARO) solenoid is ON (MAP/BARO sensor connected to barometric pressure) and the vacuum cut bypass valve is open (ON). The positive pressure test passes if the EVAP pressure sensor indicates a rise in EVAP pressure and the pressure holds until the EVAP canister vent valve is commanded open. The negative pressure leak test uses intake manifold vacuum to test the system. During the negative pressure leak test the EVAP canister purge valve is open (ON), the EVAP canister vent valve is closed (ON), and the MAP/BARO solenoid is OFF (MAP/BARO) sensor connected to manifold absolute pressure). The negative pressure test passes if the EVAP pressure sensor indicates a decrease in EVAP pressure equal to the pressure indicated from the MAP/BARO sensor and the pressure holds until the EVAP canister vent valve is commanded open.
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