P0456-EVAP PURGE SYSTEM SMALL LEAK (CONTINUED)
For the Engine circuit diagram (Refer to 9 - ENGINE - SCHEMATICS AND DIAGRAMS).
For a complete wiring diagram Refer to Section 8W.
Theory of Operation
The theory behind the Natural Vacuum Leak Detection (NVLD) is adherence to the Ideal Gas Law. Pressure in a
sealed vessel will change linearly as a function of the temperature of the gas in the vessel. Even small leaks will
allow the pressure in the vessel to come to equilibrium with the atmospheric pressure. After key-off and a calibrated
amount of time, cool down from operating temperatures or diurnal ambient temperature the system pressure will
force the system to go negative or draw a vacuum if there is no leak. When the vacuum level reaches 1” H2O (0.25
KPA) the NVLD vacuum switch closes. This sends a signal to the NGC freezing a timer and registering a pass. If
a switch closure is not detected an assessment of leak size will be made.
•
When Monitored:
Ignition off. Fuel Level less than 88%. Ambient Temperature between 4°C and 43°C (39°F and 109°F) Close
Loop fuel system.
•
Set Condition:
Due to temperature changes, a vacuum is created in the fuel tank and EVAP system. With the EVAP system
sealed, the PCM monitors the NVLD switch. If the NVLD switch does not close within a calibrated time allow-
ance, an error is detected by the PCM. One Trip Fault. Three good trips to turn off the MIL.
Possible Causes
EVAP PURGE SYSTEM LEAK
EVAP PURGE SOLENOID
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 9 - ENGINE -
DIAGNOSIS AND TESTING).
Diagnostic Test
1.
ACTIVE DTC
NOTE: Since a hot vehicle can conceal a leak, it is best to perform this test at room temperature.
NOTE: A loose gas cap could have caused this DTC to set. Make sure gas cap is tight and in good condi-
tion. Make sure the gas cap meets OEM specifications.
Ignition on, engine not running.
With a scan tool, read DTCs.
Is the DTC active at this time?
Yes
>> Go To 2
No
>> Refer to the INTERMITTENT CONDITION Diagnostic Procedure.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
9 - 502
ENGINE - ELECTRICAL DIAGNOSTICS - 3.7L/4.7L/5.7L
DR/DH