The gas cap is one of the more commonly misdiagnosed components of a vehicle’s EVAP system. The gas cap and fuel filler neck are one of many components that must remain properly sealed. If one of the EVAP system components fails to provide an airtight seal, a check engine light code will be stored for an EVAP system leak. Aftermarket (non-OEM) gas caps are a common cause of EVAP system leaks.

The vent valve is one of the two main solenoid/valve assemblies responsible for controlling the flow of EVAP system gasses. The vent valve allows fresh air to be pulled into the EVAP system and also is responsible for sealing the system when required. If the vent valve sticks in either an open or closed position or is unable to provide a vapor tight seal a code will be stored and the check engine light illuminated.

The purge solenoid is the EVAP system component responsible for controlling the volume of fuel vapors allowed to pass into the vehicle’s intake air. Failure of the purge solenoid is often misdiagnosed as its malfunction can result in codes that do not point directly to the EVAP system or its components. When off, if the purge solenoid does not completely seal, a rough idle or lean fuel trim code may be experienced. The purge solenoid is electrically controlled by a duty-cycle signal that is proportional to the volume of airflow desired.

The charcoal canister is the key component of the fuel vapor system. It captures fuel vapors and is specifically designed for each make and model vehicle. The activated charcoal in the canister assembly traps un-burned hydrocarbons and stores them for release into the engine’s intake when conditions are determined appropriate by the vehicle’s computer strategy.