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What Is The Evaporative Emissions System

How does the EVAP system work?

Fuel vapors are stored inside the EVAP system until vehicle conditions are correct. When commanded, the EVAP purge solenoid opens drawing stored fuel vapors into the engine where they mix with the air/fuel mixture and are ultimately burned

Does my car have an EVAP systems?

Almost every car made in the past 50 years has some form of the EVAP system. In 1996 a federal mandate required the EVAP system test itself for system integrity.

Do I need to maintain my EVAP system?

Most EVAP systems are maintenance free. Refer to your owner's manual for more information

Common EVAP Failures

The most common failure points of EVAP systems are:

  • Leaks
  • Component failures
  • Circuit problems

The modern EVAP system tests for leaks as small as 0.020 inches, or about the size of a pin hole. For more information on EVAP failures, click here.

What happens when an EVAP system fails?

In the event of an EVAP system failure the check engine light will illuminate to indicate your vehicles increased emissions output. In rare cases the vehicle may run rough, start hard, or fill with fuel properly.

EVAP System components

  • Fuel tank: The fuel storage container. Most today are made of a composite material and contain special rollover and pressure relief valves
  • Fuel pump: The device responsible for delivering fuel to engine. Most modern fuel pumps apply at least 50 psi
  • Fuel Filler Neck: Acts as the funnel to the fuel tank allowing for fast, efficient fuel fill ups. Also responsible for recovering the fuel vapors that would be released during the refueling process
  • Fuel lines/hoses: Plastic, composite or metal lines and hoses flow fuel and vapors between different EVAP system components
  • Purge solenoid/valve: A normally closed valve mounted under the hood connected to the engine. Click here for more information
  • Vent solenoid/valve: A normally open valve typically mounted under the car allowing the EVAP system to exchange fresh air. Click here for more information
  • EVAP canister: A storage device containing activated charcoal which filters harmful hydrocarbons and stores them until the conditions are right for the engine to burn them. Click for more information
  • Fuel tank pressure sensor: The sensor used to ensure proper operation of the EVAP system. Click for more information
  • Leak detection device: A leak detection device is found on many EVAP systems and may integrate other EVAP components within its assembly
  • Engine control module: The module responsible for monitoring and controlling the EVAP system components as well as the engine.

EVAP System at rest

The EVAP system is performing its task of controlling fuel vapors whether the vehicle is idling, driving, or parked. In the rest state, not leak testing or purging the vapors into the engine, the EVAP system is able to breathe filtered air through the charcoal canister and ultimately the vent solenoid. The exchange in and out of the EVAP system accommodates for changes in temperature which results in differences in fuel vapor pressure. If the system were 100% sealed the gas tank would expand on warm days and contract on cool days, the same way a sealed gas can would in the garage.

EVAP System purging

The charcoal canister can only contain so much fuel vapor before it becomes saturated and stops doing its job. Because of this, the EVAP system needs to purge those vapors out when engine conditions are right. This timing will vary vehicle to vehicle, but the process is similar. The engine control module opens the purge solenoid allowing engine manifold vacuum to draw the stored fuel vapors and fresh air through the vapor canister into the engine. These fuel vapors and fresh air are mixed with the air/fuel mixture and burned inside the cylinder.