The evaporative emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. The fuel tank vapors are allowed to move from a sealed fuel tank, due to pressure in the tank , through a vapor pipe into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and vent valve to atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them.

At an appropriate time the powertrain control module (PCM) commands the purge valve ON, open allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent valve OFF, open, fresh air will be drawn through the vent valve and vent line to the EVAP canister. The fresh air is drawn through the canister pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the purge pipe and purge valve into the intake manifold to be consumed during the normal combustion process.

The PCM uses several tests to determine if the EVAP system is leaking.

This tests for large leaks and blockages in the EVAP system. The PCM will command the EVAP vent valve ON, closed and the purge valve and the vacuum pressure switching solenoid ON, open with the engine running. This allows engine vacuum into the EVAP system. The PCM then commands the purge valve OFF, closed sealing the system. The PCM then monitors the fuel tank pressure (FTP) sensor signal in order to determine if there is vacuum decay. A rise or rapid increase in pressure indicates there is a large leak in the evaporative emission system. If the PCM detects the vacuum level was not achieved and vacuum decay was evident for two consecutive trips, the Malfunction Indicator Lamp (MIL) will be illuminated, and a DTC P0442 will set.

If the large leak test passes the PCM will continue to monitor the fuel tank pressure (FTP) sensor signal in order to determine if there is vacuum decay. A slight increase or rise in pressure indicates there is a small leak in the evaporative emission system. The EVAP control system is required to detect evaporative emission fuel system leaks as small as 0.5 millimeter (0.020 inch) between the fuel fill cap and the purge valve. If the PCM detects vacuum decay was evident for two consecutive trips, the MIL will be illuminated, and a DTC P0456 will set.

If the EVAP purge valve does not seal properly, fuel vapors could enter the engine at an undesired time, causing driveability concerns. The PCM tests for this by commanding the vent valve closed and the pressure switching valve open causing a vacuum, in the entire EVAP system. The PCM continues to operate the purge valve until the vacuum is lowered to a specific point, at which time the purge valve is commanded closed. The PCM monitors the fuel tank pressure (FTP) sensor. If the PCM does not detect a drop in vacuum , or the vacuum increased beyond the specified limit, for two consecutive trips, the MIL will be illuminated, and a DTC P0441 will set.

If the EVAP vent system is restricted the fuel vapors will not be properly purged. When the vapor pressure rises to a specified point, the PCM commands the vent valve OFF, OPEN, and monitors the fuel tank pressure (FTP) sensor signal. Vacuum will decrease rapidly because of the air allowed into the system. No decrease in vacuum or a decrease below a specified rate indicates a restriction on the air inlet side. If the PCM does not detect a decrease in vacuum, or the vacuum level does not change for two consecutive trips, the MIL will be illuminated, and a DTC P0446 will set.

The EVAP pressure switching solenoid is OFF, closed during engine warm up. with the pressure switching solenoid closed, vacuum pressure is blocked from entering the fuel tank and pressure in the fuel tank will rise. The FTP sensor will see a significant decrease in pressure in the tank only when the pressure switching solenoid is ON, OPEN, during a purge event. If the pressure switching solenoid malfunctions during this portion of the EVAP system operation, a DTC P0442 and DTC P0441 can set.

During the DTC P0446 portion of the EVAP system diagnostic the pressure switching solenoid is ON. This enables the FTP sensor to see a rapid decrease in vacuum, when the air from the EVAP vent solenoid enters the tank. When the PCM commands the pressure switching solenoid OFF, the FTP sensor should see the vacuum pressure stabilize and not continue to decrease. If the FTP sensor still sees a significant decrease in vacuum, the PCM concludes the solenoid did not close and sets a DTC  P0446.

The EVAP control system consists of the following components:

The EVAP canister is filled with carbon pellets used to absorb and store fuel vapors. Fuel vapors are stored in the EVAP canister until the PCM determines that the vapor can be consumed in the normal combustion process.

The EVAP canister purge valve controls the flow of fuel vapors from the EVAP system, to the intake manifold. The PCM will change the duty ratio cycle of the purge valve, controlling the purge flow volume. Purge flow volume is determined by manifold pressure, and the purge valves duty ratio cycle. Atmospheric pressure is allowed into the canister to ensure that purge flow is consistently maintained when ever purge vacuum is applied to the canister. The purge valve will also be opened during some portions of the EVAP system testing, allowing engine vacuum to enter the EVAP system.

The EVAP canister vent valve controls the flow of fresh air that passes through the EVAP canister. The vent valve is normally open. The PCM will command the EVAP canister vent valve closed during some EVAP tests, in order to seal the EVAP system for leak diagnosis.

The EVAP pressure switching solenoid opens the evaporative line between the fuel tank and the EVAP canister. When the EVAP pressure switching solenoid is closed, air is blocked from entering the tank side of the system.

The Fuel Tank Pressure (FTP) Sensor, measures the difference between the fuel tank and outside air pressure. The PCM provides a 5-volt reference and a ground to the FTP sensor. The FTP sensor provides a signal voltage back to the PCM that can vary between 0.1 and 4.9 volts. As the fuel tank pressure increases, the FTP sensor voltage decreases, as the fuel tank pressure decreases the FTP sensor voltage increases. High pressure = high voltage. Low pressure vacuum = low voltage.

The EVAP service port is located in the EVAP purge pipe, between the EVAP purge valve and the EVAP canister. The service port is identified by a green colored cap.