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Evaporative System



Evaporative Systems

The Evaporative Emission Control System (EVAP) is used to prevent gasoline vapors from escaping into the atmosphere from the fuel tank and fuel system. The EVAP system usually requires no maintenance, but faults can turn on the Check Engine light and prevent a vehicle from passing an OBD II plug-in emissions test.

The OBD II EVAP monitor on 1996 and newer vehicles runs diagnostic self-checks to detect fuel vapor leaks, and if it finds any (including a loose or missing gas cap), it will set a fault code and turn on the Check Engine light. However, the EVAP monitor only runs under certain operating conditions. This may create a problem for the vehicle owner if his vehicle must be given an OBD II plug-in emissions test and the monitor has not completed.

Sealing the fuel tank is not as simple as it sounds. For one thing, a fuel tank must have some type of venting so air can enter to replace fuel as the fuel is sucked up the fuel pump and sent to the engine. If the tank were sealed tight, the fuel pump would soon create enough negative suction pressure inside the tank to collapse the tank. On older EVAP systems, the tank is vented by a spring-loaded valve inside the gas cap. On newer vehicles, it is vented through the EVAP canister.

The major components of the evaporative emission control system include:

  • Fuel tank, which has some expansion space at the top so fuel can expand on a hot day without overflowing or forcing the EVAP system to leak.
  • Gas cap, which usually contains some type of pressure/vacuum relief valve for venting on older vehicles (pre-OBD II), but is sealed completely (no vents) on newer vehicles (1996 & newer). NOTE: If you are replacing a gas cap, it MUST be the same type as the original (vented or nonvented).
  • Liquid-Vapor Separator, located on top of the fuel tank or part of the expansion oerflow tank. This device prevents liquid gasoline from entering the vent line to the EVAP canister. You do not want liquid gasoline going directly to the EVAP canister because it would quickly overload the canister's ability to store fuel vapors. The liquid-vapor separator is relatively trouble-free. The only problems that can develop are if the liquid return becomes plugged with debris such as rust or scale from inside the fuel tank; if the main vent line becomes blocked or crimped; or if a vent line develops an external leak due to rust, corrosion, or metal fatigue from vibration.

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