What Is An Evaporative Emissions Control System

The evaporative emissions control system for an automobile is attached to the fuel tank. Usually hidden inside one of the rear wheel wells or in a cavity of the body design near the fuel tank the evaporative emissions control system plays an important role in emissions control.

Over time, as fuel sits in the fuel tank of your car the process of evaporation loses combustible gas vapors from the liquid fuel. Without a proper system to trap and store these gas vapors, they are emitted into the atmosphere where they are considered a dangerous pollutant. The evaporative emissions control system is the system that traps and stores these combustible gas vapors to be reintroduced through the intake system later.

The evaporative emissions control system consists of six basic parts. The six basic parts that I am referring to are intake system, fuel tank, vapor canister, vapor restriction, purge control valve and the pressure/vacuum relief filler cap.

Here is a simple overview of the evaporative emissions control system of your car: As the liquid fuel in the fuel tank evaporates, it is restricted to the fuel tank until the fuel tank has reached a predetermined internal pressure. When the predetermined internal pressure is reached or exceeded the vapor restriction will then allow the combustible gas vapor to pass through a line attached from the fuel tank to the vapor canister. There are three types of purge control systems, throttle body position, vacuum valve modulated flow or ECM (Electronic Control Module) activated solenoid valve. Depending on which one of these three types of purge control systems your vehicle is equipped with, when the engine has reached or exceeded idle speed the vapor canisters purge control valve will open up from applied vacuum pressure. This will allow all or some of the combustible vapors to be released from the vapor canister into the intake system. When either your engine has used up the fuel in the fuel tank or it has evaporated away, you will need to open the pressure/vacuum relief filler cap and replenish the fuel in the fuel tank at a nearby gas station.

As the fuel is burned, there are four other standard emissions control systems on U.S. built cars. The other four standard emissions control systems are the catalytic converter, the PCV valve, the EGR valve and the air injection system.

If you are not sure if your evaporative emissions control system of your car is functioning properly or within manufacturer recommended specifications you must have the evaporative emissions control system diagnosed as soon as possible.