Evaporative System Operation

The Evaporative Emissions System is designed to manage fuel vapors. The Powertrain Control Module (PCM) monitors the performance of the Evaporative Emission System. The system uses the principle of Natural Vacuum Leak Detection (NVLD) to determine the integrity of the system. When fuel vapors build and exit the Fuel Tank they are routed and stored in the Charcoal Canister. Fuel vapors entering the Charcoal Canister are absorbed by the charcoal filter until they can be drawn into the Intake Manifold during purging and burned by the engine.

The two main areas being monitored are the integrity of the system against leaks and the ability of the system to purge fuel vapor from the canister to the Intake Manifold.

• The basic strategy used for leak testing is that in a sealed system  , pressure will naturally increase or decrease in relation to temperature. The Evaporative System Integrity Monitor (ESIM) Switch is used for all leak fault detection.  There is a vacuum actuated switch that closes when the vacuum reaches a calibrated threshold. If the switch closes it indicates that the system is not leaking.
• The basic strategy used for purge performance testing is that as flow through the system increases, so does the pressure drop in the system. The Fuel Tank Pressure (FTP) Sensor is used for purge fault detection.  The PCM monitors the Fuel Tank Pressure Sensor and looks for increasing vacuum in the Fuel Tank with increasing purge flow. Conversely, it looks for decreasing vacuum in the Fuel Tank with decreasing purge flow.

For information on how the Powertrain Control Module monitors and performs diagnostics on the system. Refer to EVAP EMISSIONS PCM DIAGNOSTICS - NON TURBO  .