Evaporative Emission Control: Notes

WARNING: This page is about a different car, the 2003 Pontiac Grand Prix, 2003 Chevrolet Malibu, and 2003 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.

The Evaporative Emission (EVAP) control system limits the fuel vapors from escaping into the atmosphere. The EVAP transfers the fuel vapors from the sealed fuel tank to an activated carbon, or charcoal storage device the EVAP canister. The EVAP canister stores the vapors until the engine is able to use the extra fuel vapor. When the engine is able to use the extra fuel vapor, the intake air flow purges the fuel vapor from the carbon element and then the normal combustion process consumes the fuel vapor. The system is required in order to detect the evaporative fuel system leaks as small as 0.04" (1.0 mm) between the fuel filler cap and the EVAP canister purge valve. The system can test the evaporative system integrity by applying a vacuum signal, ported or manifold, to the fuel tank in order to create a small vacuum.

The Powertrain Control Module (PCM) then monitors the ability of the system to maintain the vacuum. If the vacuum remains for a specified period of time, then there are no evaporative leaks, and a PASS is reported by the PCM. If there is a leak, the system either will not achieve a vacuum, or a vacuum cannot be maintained. Usually a fault can only be detected after a cold start with a trip of sufficient length and driving conditions to run the needed tests. The enhanced evaporative system diagnostic conducts sub-tests to detect the fault conditions. If the diagnostic fails a sub-test, the PCM stores a Diagnostic Trouble Code (DTC) to indicate the type of fault detected.