An explanation of your vehicle's evaporative emissions system

If you are reading this sheet, chances are your car or truck's CHECK ENGINE light has come on and set a code for a problem in the evaporative emission (evap) system. We put this together to explain WHY this code may have set and to explain what the evap system does.

Why might the CHECK ENGINE light have come ON?

1. Was the gas cap tight? If the gas cap is loose the light will come on (see below).
   
   
2. Did you overfill the gas tank? Always stop pumping gas when the pump first clicks off. Overfilling the tank can set this code and also cause damage to other parts of the emission system.
   
   
3. Did you fill the gas tank while the engine was running? Not only is this a dangerous habit (an explosion hazard), but it might also set this trouble code.
   
   
4. There could actually be a fault in the evap system. This could be a problem with one of the system's sensors, actuators (little electronic valves), or in the hoses and steel lines that connect all these sensors and actuators.

Why does my car/truck have an evap system?

The gas tank in most cars/trucks holds between 15 and 20 gallons of fuel. When the gas tank gets low (say down to 5 gallons), the remaining "empty" part of the gas tank is actually filled with air. This air is saturated with gasoline fumes.

When you remove the gas cap to fill the tank with gas, this gas fume saturated air is pushed out of the gas tank and into the surrounding air. This "polluted" air is a major cause of smog. The problem is particularly bad in a valley like we live in around York, and is even worse on hot, humid days.

To eliminate this pollution source, your car has an evap system. It is designed to seal tight once the tank is full and the gas cap is closed tight! As the gas in the tank is used, the evap system pulls the gasoline saturated air up into the engine where it is slowly added into the engine to be burned. (This is a simplified explanation.)

The evap system must seal tight. All the sensors and actuators must work. Otherwise a code will set in the computer system and light the CHECK ENGINE light.

Loose gas caps are the most common reason a trouble code will set in the computer system. Please make sure to completely tighten your gas cap and follow the other suggestions above. If the CHECK ENGINE light continues to come ON and a code continues to set for a problem in the evap system, we will need to perform further diagnostics.

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