Description

The evaporative emission control system measures small pressure changes (on the order of 0.01 psi) to detect if gasoline vapor may be leaking into the atmosphere. OBD-II is very demanding of the evaporative emission control system, which must be able to detect leaks as small as 1 mm (0.040 in.). If a leak is detected, the "Check Engine" light or Malfunction Indicator Light (MIL) will illuminate and a diagnostic code will set. This TSB describes the evaporative emission control system operation, leak check test procedure, diagnostic trouble codes and diagnostic procedures. The evaporative emission control system is made up of the following components:

Charcoal Canister: Absorbs fuel vapors and stores them until they are purged into the intake manifold.

Purge Control Valve: An electrically operated solenoid valve that applies engine vacuum to the canister to remove stored fuel vapors.

Canister Close Valve (CCV): An electrically operated solenoid valve that closes the canister fresh air supply and seals the system. Only used for test purposes.

Two Way Valve: Controls flow of air into and out of the canister. Acts as a pressure relief if the system pressures get too high.

Fuel Tank Pressure Sensor (FTP) or Differential Pressure Sensor (DPS): Measures pressure in the fuel tank. Used by the Powertrain Control Module (PCM) to detect purge valve operation and leaks in the evaporative system. Located near the fuel filler area in the wheel well.

Fuel/Vapor Separator: Returns raw fuel to the tank and allows only vapors to flow to the canister. Raw fuel may quickly saturate the charcoal in the canister and reduce its storage capacity.

Fuel Tank: Can be completely sealed from outside air to prevent unwanted fuel vapors from being vented into the air.