How An Evaporative Emissions Control System Works

The evaporative emissions control system is part of the emissions control system that works to prevent harmful hydrocarbons and hazardous gases to be released from the operation of your vehicle. The evaporative emissions control system is not just in use when your vehicle is running, it also works when your vehicle is turned off.

Gas is a very easily evaporated substance and tends to turn from a liquid into a vapor at lower temperatures that many other substances. Until vehicles were equipped with the evaporative emissions control system component, there was a significant amount of liquid gas in the fuel tank that simply turned into vapor and was vented or released into the air. It was estimated that before 1970 when evaporative emissions control system programs were put in place, about twenty percent of all hydrocarbons released into the air were from gasoline evaporation from vehicle gas tanks.

The evaporative emissions control system turns what used to be an open system into a closed system, trapping the vapor in a canister. Both the gas tank as well as the carburettor vent into the canister that is filled with charcoal. When the vapor from the gasoline moves through the canister it sticks to the charcoal, which is a type of filter. The tiny vapor particles are held into the charcoal but when the vehicle is started again the suction produced by the engine will pull the vapors back into the engine. This vapor is then burned in the engine as part of the combustion of the motor, resulting in a cleaner burning engine.

In order for the evaporative emissions control system to work correctly, the system must be completely sealed. This includes a sealing gas cap to the outside of the vehicle. If the seal is damaged in some way, the system does not work properly and will trigger the “check engine” light to come on. Once this happens, taking the vehicle to a technician who can then check the on-board diagnostic system will confirm that there is a gas cap seal problem. Simply replacing the gas cap typically fixes the problem and make sures that the evaporative emissions control system is working correctly. Another common problem is a sticky valve to the canister, which will result in an improper air fuel ratio and poor vehicle performance. The valve can also stick open, result in incorrect fuel mixtures as well. Finally the filter to the charcoal canister can also become plugged. This is typically seen as a decrease in power and acceleration when the vehicle is being driven. Even if the engine light does not come on, this filter and the valve should be checked during all tune-ups.