Enhanced EVAP System Diagnosis

As required by law, the diagnostic routine must be able to detect a leak as small as 0.040 inches in diameter. 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.

ENHANCED EVAP SYSTEM DIAGNOSTIC
The Enhanced Evaporative System Diagnostic is a precise series of operational sub-tests on the system. In some cases, the failure of only one sub-test will end all evaporative system testing. However, in many cases, the on-board system will conduct a series of sub-tests before storing any DTCs. A brief description of each of the sub-tests follows.

Vacuum Test at System Power-up

NOTICE: The diagnostic requires approximately 500 ms in order to run a completion. If the ignition switch is turned to the crank position too quickly, the diagnostic may not complete the test.

This test is conducted during a cold start (ECT = IAT±5°C) with a fuel level that is between 85% and 15% and an engine that is not cranking.

Given the correct conditions at the system power-up, the PCM commands the vent valve open and purge valve closed. The fuel tank pressure sensor should indicate no pressure or vacuum during these conditions. If a pressure or vacuum is indicated, the [1][2]EVAP diagnostic is completed for that trip and a DTC P0446 is stored. If the conditions are not correct, this sub-test is skipped for this trip.

Weak Vacuum Test - Stage 1
The Weak Vacuum Test runs only if the previous test did not report a Pass. For this test, the vent valve is commanded closed during normal purge operation. If sufficient vacuum does not develop, the diagnostic fails and the control module stores a DTC P0440. If a sufficient vacuum develops, the Small [1][2]EVAP Leak Test runs. If the proper driving conditions are not met this test will be bypassed.

Small [1][2]EVAP Leak Test

NOTICE: This test may run up to three times prior to reporting a failure. A P0442 indicates a small leak anywhere between the fuel fill cap to, but not including, the purge solenoid.

The Small Leak Test runs immediately following a passing Weak Vacuum Test. While the vacuum is still present in the fuel tank, the vent valve and purge valve are both commanded closed in order to seal the system. With the system closed, the PCM monitors the trapped vacuum in the system through the fuel tank pressure sensor. If the vacuum decay rate is too fast, a small leak is indicated and a DTC P0442 is stored. Otherwise, DTC P0442 is passed and the Excess Vacuum Test - Stage 2, is run.

Excess Vacuum Test - Stage 2

IMPORTANT: This test is the same as the Stage 1 Excess Vacuum Test except for different driving conditions under which the test is conducted.

This test allows the normal purging and commands the vent valve open. If a high amount of vacuum occurs, this indicates that a restriction has occurred in the canister vent or the canister vent valve. If an excessive vacuum develops, a DTC P0446 stores. If an excessive vacuum does not develop, a DTC P0446 passes and the Purge Valve Leak Test runs.


Weak Vacuum Test - Stage 2

IMPORTANT: This test enables the malfunction indicator lamp (MIL) on the next key up if a Fail is reported and extinguishes the MIL on the next key up when a Pass is reported. When the MIL is extinguished, the executive also erases the freeze frame and the failure record buffer for this DTC (check for missing fuel fill cap). This test also runs on a hot start when all others are aborted if a prior failure exists.

This test is used in order to determine if the system can develop a sufficient vacuum. During a normal [1][2]EVAP purging, the vent solenoid is commanded closed and the fuel tank pressure sensor is monitored by the PCM. If sufficient vacuum is developed, a DTC P0440 passes. Otherwise, the Enhanced Evaporative System Diagnostic Test ends.

Purge Valve Leak Test
The Purge Valve Leak Test runs only if there was a cold start this ignition and the Weak Vacuum Test - Stage 1 test did not fail. This test monitors the purge solenoid for leaks. During the test, both the vent and the solenoid close. If a vacuum develops, the purge valve is leaking and a DTC P1441 is set.

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

RESULTING DIAGNOSTIC TROUBLE CODES
The PCM conducts several intrusive tests on the Evaporative Emissions System and separates faults into one of the following fault categories:

DTC P0440 - Large Leak Detected.
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, fuel filler neck, stuck-closed purge solenoid, stuck-open purge solenoid, disconnected or damaged purge line or vent solenoid hose will set a P0440.

DTC P0442 - Small Leak Detected.
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 system is failing, 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 greater than a calibrated amount within a specific amount of time, a P0442 will be stored and the Service Engine Soon (MIL) will be illuminated.

DTC P0446 - Canister Vent Solenoid Blocked.
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) will be illuminated. The P0446 can also be 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.

DTC P1441 - [1][2]EVAP 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 fuel tank pressure (vacuum) is greater than a calibrated amount within a specific time, a P1441 will be stored and the Service Engine Soon (MIL) will be illuminated. A leaking purge solenoid or incorrect routing of lines (vacuum and purge lines reversed) will set a P1441.

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 P0443 - Canister Purge Solenoid Circuit Fault.
^ DTC P0449 - Canister Vent Solenoid Circuit Fault.
^ DTC P0452 - Tank Pressure Sensor Circuit Fault - Low Voltage.
^ DTC P0453 - Tank Pressure Sensor Circuit Fault - High Voltage.

Refer to the appropriate DTC chart for further explanation.