Theory of Operation

Evaporative systems are not new to the automotive industry. The first systems were introduced in the U.S. industry in the 1970's as part of the overall scheme to reduce vehicle hydrocarbon emissions. They had not changed appreciably until the U.S. government mandated as part of the OBD II diagnostics a tightened control on hydrocarbon emission.

Enhanced EVAP systems are designed to 1) reduce emissions from the fuel system both while the vehicle is operating or parked and 2) continually monitor the system for leaks and to activate the Service Engine Soon lamp if a problem is detected. As required by law, the diagnostic routine must be able to detect a leak as small as 0.040 inches. The integrity of the entire vapor handling portion of the fuel system including the purge solenoid, purge lines, fuel tank, EVAP canister, vent line, vent solenoid, fuel filler pipe, and fuel cap is continually tested.

Evaluation of system integrity is the function of the PCM diagnostic routines. The PCM monitors the ability of the system to maintain vacuum. If the vacuum remains for a specified period of time, then there are no evaporative leaks and a PASS is reported to the diagnostic executive. If there is a leak, the system either will not achieve a specific vacuum, or a vacuum cannot be maintained. A fault can only be detected after a cold start with a trip of sufficient length and driving conditions to run the needed tests.

The EVAP purge solenoid allows intake manifold pressure (vacuum) to purge the stored vapors in the activated carbon ([1][2]EVAP) canister. The PCM supplies a ground to energize (open) the normally closed purge solenoid which allows vacuum to purge the canister. The ground is Pulse Width Modulated (PWM - pulsed ON and OFF several times per second) to control the flow of purge vapors from the canister. The PCM calculates a PWM duty cycle for canister purge flow control using the following operating parameters: engine load, throttle position, coolant temperature, and ambient temperature.

The [1][2]EVAP vent solenoid is used to seal the EVAP system during the EVAP leak diagnostic tests. The fuel tank pressure sensor is used to monitor system pressure (vacuum) during the leak tests.

The PCM conducts several intrusive tests on the evaporative emission system and separates faults into one of the following fault categories:
^ DTC P0440 - EVAP system. The PCM commands the vent solenoid closed and the purge solenoid to a fixed duty cycle while monitoring the fuel tank pressure sensor. If the system is not capable of creating a calibrated vacuum in the system within a specific amount of time, the PCM logs a pending fault flag and awaits the next test results. A warm test is done if the cold test fails. The warm test can only PASS the diagnostic. A PASS will extinguish the Malfunction Indicator Lamp (MIL) immediately. A leaking fuel cap or fuel filler neck, stuck closed purge solenoid, stuck open vent solenoid, or a disconnected or damaged purge line or vent hose will set a P0440.
^ DTC P0442 - EVAP system. After passing the large leak diagnostic, the PCM commands the vent solenoid closed and the purge solenoid to a fixed duty cycle while monitoring the fuel tank pressure sensor. After a calibrated vacuum level is achieved and the PCM then turns off the normally closed purge solenoid, continues to hold the vent solenoid closed, and monitors the decay of vacuum in the system. If the decay is excessive, a P0442 will be stored and the Service Engine Soon (MIL) is illuminated.
^ P0446 - [1][2]EVAP vent solenoid valve system. The PCM commands the vent solenoid closed and the purge solenoid to a fixed duty cycle while monitoring the fuel tank pressure sensor. The PCM then releases the normally open vent solenoid and expects a rapid decay in system pressure. If the pressure decay is less than a calibrated value in a given amount of time, a P0446 will be stored and the Service Engine Soon (MIL) is illuminated. The P0446 can also set anytime the vent solenoid is being commanded off (open) and the vacuum in the tank exceeds a calibrated value. A stuck closed vent solenoid, a restricted vent hose, or a blocked canister will set the P0446.
^ P1441 - EVAP system flow during non-purge. The PCM commands the vent solenoid closed and the purge solenoid to a 0% duty cycle (no flow) while monitoring the fuel tank pressure sensor. The PCM expects system pressure to remain constant. However, if system pressure (vacuum) increases during the test, a continuous purge flow is indicated. If pressure (vacuum) is greater than a calibrated amount within a specific time, a P1441 will be stored and the Service Engine Soon (MIL) is illuminated. A leaking purge solenoid or incorrect routing of lines (vacuum and purge lines reversed) will set the P1441.

In addition to these intrusive tests, the PCM monitors the control circuits to the vent and purge solenoids and the tank pressure circuitry for possible faults. These faults are as follows:
^ DTC P0452 - [1][2]EVAP fuel tank pressure sensor circuit low voltage.
^ DTC P0453 - [1][2]EVAP fuel tank pressure sensor circuit voltage.

Refer to the appropriate DTC charts for further explanation.