When we think of controlling vehicle emissions, our first thoughts are usually about cleaning up tailpipe emissions. However, with today’s gasoline powered vehicles, if left unchecked, the gasoline fumes evaporating from the fuel tank and engine are also a major source of hydrocarbon pollution. That means if the fuel system is open to the atmosphere, it can pollute 24 hours a day even if the engine is not running. The Evaporative Emission Control System (EVAP) is used to prevent gasoline vapors from escaping into the atmosphere from the fuel tank and fuel system at all times.

EVAP systems have been required on gasoline powered vehicles since the 1970s. A typical system consists of the fuel tank, an EVAP vapor storage canister full of charcoal, valves, hoses and a sealed fuel tank gas cap. The EVAP system is designed to stop fuel system fumes from leaking directly into the atmosphere. Vent lines from the fuel tank pass vapors to the vapor canister, where they are trapped and stored until the engine is started. When the engine is warm and the vehicle is going down the road, the PCM then opens a purge valve allowing the vapors to be drawn from the storage canister into the intake manifold. The fuel vapors are then burned in the engine along with the air/fuel mixture.

EVAP systems are active systems that require very little maintenance. On pre-OBD II vehicles the PCM software had no method to verify that the EVAP system was working as designed. The only time this system was paid attention to was when there was a failure of a component that allowed for fuel from the tank to be drawn into the engine creating an extremely rich condition or when a component failure created a vacuum leak creating a lean mixture condition. Both of these issues were typically diagnosed as performance issues, not emissions. On OBD II equipped vehicles the PCM monitors the fuel system for fuel vapor leaks to make sure no hydrocarbons are escaping into the atmosphere. The EVAP monitor does two things: it verifies there is airflow from the EVAP canister to the engine, and that there are no leaks in the fuel tank, EVAP canister or fuel system vapor lines.

The PCM runs an EVAP diagnostic monitor on OBD II equipped vehicles under certain driving conditions to detect fuel vapor leaks, and if it finds any it will set a DTC fault code and illuminate the Check Engine light. However, the EVAP monitor only runs under very precise operating conditions including engine running characteristics, ambient temperature requirements and fuel tank volume requirements. The diagnostic routine run by the PCM is capable of detecting extremely small leaks which may be difficult to isolate and repair. Let’s take a deeper look at some of the evaporative emissions components and their functions.

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