Evaporative Emissions System: Testing and Inspection

Circuit Description
The evaporative (EVAP) emission system is checked by applying vacuum to the EVAP emission system and monitoring for a vacuum decay. The powertrain control module (PCM)/engine control module (ECM) monitors the vacuum level through the fuel tank pressure sensor signal. At an appropriate time, the EVAP emission canister purge valve and the EVAP emission vent solenoid are turned 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 emission canister purge valve is turned off, sealing the system. A leak is detected by monitoring for a decrease in vacuum level over a given time period, when all other variables remain constant. A leak, blockage or faulty component in the system will cause a Diagnostic Trouble Code (DTC) to be set.

Diagnostic Aids
Check for the following conditions:
- Poor connection at the PCM/ECM. Inspect harness connectors for the following conditions:
- Backed-out terminals.
- Improper mating.
- Broken locks.
- Damaged terminals.
- Poor terminal-to-wire connection.
- Damaged harness. Inspect the wiring harness for damage. If the harness appears to be OK, observe the Fuel Tank Vacuum Pressure display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the display will indicate the location of the fault.
- Incorrect purge or vacuum source line routing. Verify that the source vacuum and purge lines to the EVAP emission canister purge valve are not switched.

Steps 1 - 11:

Steps 12 - 15:

EVAP CONTROL SYSTEM DIAGNOSIS