The Correct Answer and Explanation is:
The correct answer is C) Canister vent valve.
An automotive Evaporative Emission Control (EVAP) system is designed to prevent harmful gasoline vapors from the fuel tank from escaping into the atmosphere. The system uses two primary valves to manage this process: the canister purge valve and the canister vent valve. Understanding their default, or “normal,” states is key to diagnosing the system.
The canister vent valve, also known as the vent solenoid, is normally open. Its primary function is to allow fresh air to enter the charcoal canister. When the engine is off, this open path allows fuel vapors expanding in the tank to flow into the canister, where they are absorbed by activated charcoal. When the engine is running and the system decides to purge these stored vapors, the vent valve remains open. This allows fresh air to be drawn through the canister, helping to pull the fuel vapors out and into the engine to be burned. The only time the canister vent valve is commanded to close is when the vehicle’s computer runs a diagnostic leak test on the EVAP system. By closing both the vent and purge valves, the system is sealed, and the computer can monitor for pressure changes to detect leaks.
In contrast, the canister purge valve is normally closed. This valve is located between the charcoal canister and the engine’s intake manifold. It acts as a gate, controlling when the stored fuel vapors are drawn into the engine. If it were normally open, it would create a constant vacuum leak, leading to poor engine performance, a rough idle, and incorrect air fuel ratios. The engine’s computer only opens the purge valve under specific operating conditions, such as when the engine is warmed up and running above idle speed. Therefore, its normal, unpowered state is closed.
