The enhanced EVAP system is used to limit unburned fuel vapors, or hydrocarbons, from escaping into the atmosphere. Since fuel evaporation in the fuel tank is a normal process, it is necessary to accumulate the unburned fuel vapors so they can be purged into the intake manifold for combustion. The EVAP system consists of the following:

When fuel from the fuel tank evaporates, the vapors flow though the fuel level vent valve (FLVV), located on the top of the fuel tank, to the EVAP canister. The EVAP canister is an active carbon fiber storage container that collects fuel vapors and prevents them from entering the atmosphere. The EVAP canister is vented to the atmosphere through the EVAP vent solenoid. The EVAP vent solenoid is a normally open valve that only closes during parts of the EVAP diagnostic test.

When the ECM goes into purge mode, drawing fuel vapors into the intake manifold, it will pulse width modulate the EVAP purge solenoid by grounding the EVAP purge solenoid circuit. As the EVAP purge valve opens and closes, manifold vacuum draws fuel vapors from the EVAP canister.

The ECM monitors the following EVAP system electrical components for open, low, and high voltage faults:

As part of the On-Board Diagnostics second-generation (OBD II) requirements, the government mandates that all automobile manufactures detect a 0.51 mm (0.02 in) or greater diameter leak in the EVAP system. To comply, the ECM monitors the EVAP system for large and small leaks and whether the EVAP purge and vent solenoid valves are stuck closed or open.

Important: The following flowchart consists of a series of tests run by the ECM, which describes the entire EVAP DIAGNOSTIC TEST. Each step the ECM performs is dependent on the accuracy of the fuel tank pressure sensor. The FUEL TANK PRESS. SENSOR reading on the scan tool with the ignition ON and the engine OFF and the fuel cap removed should read between 2.35-2.85 volts.

When the MIL is commanded ON and an EVAP DTC is set, there are certain ordered steps that should be taken to ensure correct diagnosis.

Important: If any of the following DTCs are set, the ECM will NOT run the EVAP diagnostic leak test.

1. Diagnose any hard circuit fault fuel tank pressure (DTCs P0451, P0452 or P0453), EVAP purge solenoid (DTCs P0443, P0444 or P0445), or EVAP vent solenoid (DTCs P0447, P0448 or P0449) DTCs first.
2. Using the scan tool, monitor FUEL TANK PRESS. SENSOR with the ignition ON and the engine OFF and the fuel cap removed. The voltage should be between 2.35-2.85  volts.
3. Reinstall the fuel cap.
4. If DTC P0440/P0496 or P0446 is set, diagnose that DTC next.
5. If DTC P0455 or P0442 is set, diagnose that DTC next. A leak in the system must be present if 1 of these 2 DTCs are set. Use the EVAP System Leak Tester to locate the leak.

Important: The EVAP System Tester has the ability to serve 2 functions. It can induce smoke in the EVAP system to determine where a small or large EVAP system leak exists, and it can verify the integrity of the system after a fix has been performed by the use of a flowmeter test. The flowmeter test introduces nitrogen into the system to determine if a floating device is above a certain point. If the floating device is above a certain point, then nitrogen flow is present and a 0.51 mm (0.02 in) leak or greater is present.

Important: Ensure that the vehicle underbody temperature is similar to ambient temperature, and allow surrounding air to stabilize before starting the diagnostic procedure.

Smoke Test

1. Turn the ignition ON, with the engine OFF.
2. Using the scan tool, seal the EVAP system, using the EVAP PURGE/SEAL special function.
3. Connect the Evaporative Emission System Tester (EEST) red battery clip to the positive (+) battery terminal. Connect the black battery clip to the chassis ground.
4. Connect the EEST nitrogen/smoke hose onto the EVAP service port.
5. Turn the NITROGEN/SMOKE valve to SMOKE.
6. Verify smoke has filled the EVAP system by removing the gas cap. Smoke is visible leaving the fuel fill pipe when the system is full. Reinstall the gas cap. Induce smoke for 1 additional minute.
7. Use a J 41413-SPT High Intensity White Light to inspect the entire EVAP system for exiting smoke. Introduce smoke at 15 second intervals as needed until the leak source is identified.

Flowmeter Test

1. Turn the NITROGEN/SMOKE valve to NITROGEN.
2. Connect the hose to 0.51 mm (0.02 in) test orifice on the bottom-front of the EEST.
3. Use the remote switch to activate the EEST.
4. Align the red flag on the flowmeter with the floating indicator. Use the remote switch to deactivate the EEST.
5. Remove the hose from the test orifice. Reinstall onto the EVAP service port.
6. Turn the ignition ON, with the engine OFF.
7. Using the scan tool, seal the EVAP system using the EVAP PURGE/SEAL special function.

Important: Larger volume tanks, and/or those with lower fuel levels, may require several minutes for the floating indicator to stabilize.

8. Use the remote switch to activate the nitrogen flow and fill the system. Watch the floating indicator until it stabilizes and stops dropping.
9. Compare the flowmeter stable floating indicator position to the red flag. If the indicator floats:

•	ABOVE the red flag -- UNACCEPTABLE (FAIL - Return to service information)

•	BELOW the red lag -- ACCEPTABLE (PASS - Test completed)