Theory Of Operation

The Powertrain Control Module (PCM) evaluates the complete Evaporative System for component function and vapor leaks to the atmosphere. Leak testing is performed after the ignition is turned off. The leak test is delayed for 6 hours after the ignition is turned off to allow the Fuel Tank temperature and pressure to stabilize. If the tank pressure is too high the vacuum pump inside the ELCM may not be able to pull the system into a vacuum causing a false fail.

The individual components of the Evaporative System are monitored for circuit continuity, out of range values, rationality, and proper functional response.

• The Evaporative Leak Check Module (ELCM)  is required for the OBD leak detection test. The ELCM contains the vacuum pump, a 0.020" reference orifice, pressure sensor, changeover valve, and air filter. There is a water trap that attaches to the ELCM externally. Refer to FUNCTIONAL DESCRIPTION - EVAPORATIVE LEAK CHECK MODULE (ELCM) for a more detailed description on the ELCM internal components.
• The Fuel Tank Isolation Valve (FTIV)  provides Fuel Tank vapor management, including mechanical pressure and vacuum relief. The FTIV is a normally closed device which separates the Fuel Tank from the purge vapor canister to maintain pressure or vacuum in the Fuel Tank. The FTIV is opened by the PCM:
  1. to provide controlled depressurization of the fuel tank during refueling.
  2. to manage the fuel tank vapor concentration when the engine is running in hot ambient temperatures.
  3. to provide access to the fuel tank during the EVAP System leak detection test.

  Refer to FUNCTIONAL DESCRIPTION - FUEL TANK ISOLATION VALVE (FTIV) for a more detailed description on the FTIV internal components.

• The Fuel Tank Pressure (FTP) Sensor  is located between the Fuel Tank and the Filler Neck and provides the Fuel Tank pressure or vacuum level input to the PCM for Fuel Tank vapor management. The FTP sensor is also an input to the EVAP System leak detection test.
• The Hydrocarbon Trap  is a passive device located between the purge vapor canister and the ELCM Vacuum Pump.

Courtesy of CHRYSLER GROUP, LLC

for a more detailed description of each phase of the Engine off Small Leak Monitor diagnostic performed by the PCM.  Refer to EMISSIONS CONTROL/EVAPORATIVE EMISSIONS/DIAGNOSIS AND TESTING .