P0442-EVAP PURGE SYSTEM MEDIUM 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:
Engine Running. Cold start test. Fuel Level greater than 12%. Ambient Temperature between 4°C and 32°C
(39°F and 89°F) Close Loop fuel system. Test runs when small leak test is maturing.
•
Set Condition:
The PCM activates the EVAP Purge Solenoid to pull the EVAP system into a vacuum to close the NVLD
switch. Once the NVLD switch is closed, the PCM turns the EVAP Purge solenoid off to seal the EVAP system.
If the NVLD switch reopens before the calibrated amount of time for a Medium leak an error is detected. Two
Trip Fault. Three good trips to turn off the MIL.
Possible Causes
EVAP PURGE SYSTEM LEAK
EVAP PURGE SOLENOID
NVLD SWITCH
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)
HB
ENGINE ELECTRICAL DIAGNOSTICS
9 - 479