Component Functional Description And Diagnostic Overview - OBD Vent Valve

Functional Operation:  The normally closed OBD Vent Valve is used to provide open venting to the atmosphere when energized upon command by the Powertrain Control Module (PCM). The valve is used as part of the Evaporative System diagnostics on Turbocharged engines for two purposes:

1. ESIM Switch Stuck Closed Diagnostic:  During small leak testing vacuum is often trapped in the EVAP system when the engine is turned off. The system is vented to atmosphere to relieve the vacuum and open the switch. Since the system is sealed to atmosphere through the check valves in the Proportional Purge Solenoid (PPS), the OBD Vent Valve is energized by the PCM to vent the system.
2. Purge Performance Diagnostic:  This diagnostic looks for the delta pressure change in the Fuel Tank Pressure (FTP) Sensor signal when the Purge Solenoid is energized. If vacuum is already trapped in the system it may be difficult to generate enough of a delta pressure change to obtain good separation between a passing and failing system. Because both sides of the system are tested during each drive cycle the OBD Vent Valve is used to eliminate the excess vacuum before the Purge Flow Monitor Diagnostic begins for each side.

Diagnostic Overview:  The PCM performs open circuit and short circuit diagnostics on the control circuitry for the OBD Vent Valve. Detecting a valve that is mechanically stuck open or closed is caught by other EVAP system diagnostics. A stuck open valve would cause a system leak and would fail the small leak or large leak diagnostic. A valve that is stuck closed would cause one or both of the ESIM Switch Stuck Closed or Purge Performance diagnostics to fail.