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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 1350 (Engine Control System (Troubleshooting & Diagnosis) -- 1.8L (LNK)) >> Troubleshooting >> Service Bay Test >> EVAP Purge/Seal Function

EVAP Purge/Seal Function

WARNING: This page is about a different car, the 2004 Pontiac Vibe. However, it is still accessible from the selected car via links, so may be relevant.

The EVAP purge/seal function is designed to seal the EVAP system with a vacuum applied. The test allows the technician to monitor the fuel tank pressure (FTP) sensor, for excessive vacuum decay. The purge/seal function can also alert the technician of an inoperable purge, vent, or pressure switching solenoid. When the EVAP purge/seal function is enabled, the PCM commands the vent and pressure switching solenoids ON or closed. The purge solenoid is commanded ON, open, allowing a vacuum to enter the EVAP system. Once the vacuum pressure reaches approximately -28.79 mm Hg (-15.41 in H2O), the PCM commands the purge solenoid OFF, closed, sealing the system. The EVAP system will remain sealed for 120 seconds.

In order to perform the EVAP purge/seal test with the scan tool follow the procedures below:

IMPORTANT: Ensure that the vehicle underbody temperature is similar to the ambient temperature and allow the surrounding air to stabilize before starting the diagnostic procedure. Fuel tank pressure may increase on a hot vehicle due to the thermal expansion of vapors in the fuel tank.
IMPORTANT: Fuel tank fill levels can affect the results of the diagnostic procedure. Fuel tank pressure may increase on a warm vehicle that has less than one third a tank of fuel.
  1. Run the engine to normal operating conditions.
  2. Install the scan tool.
  3. Turn ON the ignition, with the engine OFF.
  4. Remove the fuel fill cap. The FTP sensor should read near 0.0 mm Hg (0.0 in H20).
  5. Install the fuel fill cap and start the engine.
  6. Enable the EVAP purge/seal function on the scan tool.
  7. The FTP sensor reading should decrease to at least -28.20 mm Hg (-15.10 in H2O), indicating a vacuum has entered the system.
  8. Observe the FTP sensor reading until the test status changes to OFF - approximately 2 minutes. If the FTP sensor reading remains above -26.10 mm Hg (-14.01 in H2O), the test has passed.
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