Emission Control Systems: Testing and Inspection

EVAP Control System Diagnosis

Circuit Description
Applying a vacuum to the EVAP system and monitoring for a vacuum decay checks the evaporative emission system. The Control Module monitors the vacuum level through the fuel tank vacuum sensor signal. At an appropriate time, the EVAP canister purge valve and the EVAP vent solenoid turn ON, allowing the engine to draw a small vacuum on the entire evaporative emission system. After the desired vacuum level has been achieved, the EVAP canister purge valve turns OFF, sealing the system. A leak is detected by monitoring for a decrease in the vacuum level over a given time period, when all other variables remain constant. A leak, a blockage or a malfunctioning component in the system causes a DTC to set.

Test Description














The numbers below refer to the step numbers on the diagnostic table.
3. This step checks the fuel tank pressure sensor at atmospheric pressure.
4. This step determines if the EVAP system has a restriction or leak.
5. This step determines if there is a restriction in the Fuel Tank Vapor Line or Canister.
8. This step checks for a restriction in the EVAP Canister.