Principles of Operation

Evaporative Emission System, Part 1:

Evaporative Emission System, Part 2:

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, excess air (which was drawn into the fuel tank as the fuel level decreased) and fuel vapor 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 (for example not during deceleration, 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.

Vacuum Cut Valve
The vacuum cut valve is a one-way check valve which allows fuel vapors to enter the EVAP purge line, but prevents intake manifold vacuum from being applied to the fuel tank.

Vacuum Cut Bypass Solenoid
The vacuum cut bypass solenoid is installed in parallel with the vacuum cut valve on the EVAP purge line between the EVAP canister and the fuel tank. When open, the vacuum cut valve is bypassed to apply intake manifold vacuum to the fuel tank. The vacuum cut bypass solenoid generally remains closed, and is used only for EVAP system diagnosis.

Evaporative Emission System Test Port
The EVAP system has a test port located in the EVAP canister purge line. The EVAP system test port is used to connect the EVAP system leak tester, which can confirm and aid in locating an EVAP system leak.

Evaporative Emission Canister Purge Valve
The EVAP canister purge valve is controlled by the PCM. The EVAP canister purge valve controls the flow of fuel vapors from the EVAP canister to the engine intake manifold during various engine-operating modes. The EVAP canister purge valve is normally closed.

Evaporative Emission Canister Pressure Sensor
The EVAP canister pressure sensor is an input to the PCM and is used for on-board diagnosis of the EVAP system. The sensor is located in the purge line of the EVAP canister and senses changes in pressure in the EVAP canister purge line.

Evaporative Emission Canister
Fuel vapors from the fuel tank are stored in the EVAP canister. When the engine is running, the vapors are purged from the EVAP canister for combustion. OBD II vehicles sometimes use multiple canisters, depending upon the size and number of the fuel tanks used on a specific vehicle.

Evaporative Emission Canister Vent Control Valve
During the Evaporative Emission Running Loss System test monitor, Evaporative Emissions Repair verification drive cycle and the Evaporative Emission system leak test the canister vent control valve is closed to allow either a vacuum to be drawn on the fuel tank or to hold a specified pressure in the system. The canister vent control valve is normally open.

Evaporative Emission Water Separator
The water separator is attached to the EVAP canister vent control valve inlet. When the EVAP canister vent control valve is open, air is drawn into the EVAP canister through the water separator. The water separator draws water vapor from the incoming air and then vents it back to the atmosphere.

Fuel Filler Cap
The fuel filler cap is used to prevent fuel spill and to close the EVAP system to atmosphere.

Evaporative emission system monitor
The evaporative emission (EVAP) system monitor is a self-test strategy within the PCM which 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 he 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 correct 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 control 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 control valve is commanded open. The negative pressure leak test uses an 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 control 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 control valve is commanded open.

EVAP Running Loss System Test
To start the testing, conditions of stable purging and vehicle speed must be satisfied. During the first stage, the EVAP canister vent control valve is closed while the EVAP canister purge valve remains open, applying and building vacuum in the system as indicated by the EVAP canister pressure sensor. This phase checks for major leaks in the EVAP system.

In the second stage, the EVAP canister purge valve closes and the system looks for minimal decay rate in the EVAP vacuum, indicating the absence of any small EVAP system leaks.

The last stage is entered only if stage two of the leak test has failed and checks whether the failed test was due to excess vapor generation. It monitors fuel vapor generation rate. Initially the canister vent control valve is opened to equalize EVAP system pressure to atmosphere. Then the canister vent control valve is closed allowing pressure to build, if vapor generation is present in sufficient quantity. If the rate of generation is found to be too high, the EVAP running loss system leak test is aborted. If not, then a small leak is diagnosed.