EVAP System Operation
The evaporative emission (EVAP) control system limits fuel vapours from escaping into the atmosphere. Fuel tank vapours are allowed to move from the fuel tank, due to pressure in the tank, through the vapour pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapours. Excess pressure is vented through the vent line and out to atmosphere. The EVAP canister stores the fuel vapours until the engine is able to use them. At an appropriate time, the control module will command the EVAP purge solenoid ON, open, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent open, fresh air will be drawn through the vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapours from the carbon. The air/fuel vapour mixture continues through the EVAP purge tube and EVAP purge solenoid into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.
Canister Vent Restriction Test
If the evaporative emission (EVAP) vent system is restricted, fuel vapours will not be properly purged from the EVAP canister. The control module tests this by commanding the EVAP purge solenoid ON, open; and monitoring the ECM for an increase in vacuum. If vacuum increases more than a calibrated value, DTC P0446 will set.
Purge Solenoid Leak Test
If the evaporative emission (EVAP) purge solenoid does not seal properly, fuel vapours could enter the engine at an undesired time, causing driveability concerns. The control module tests for this by commanding the EVAP purge solenoid OFF, closed; sealing the system, and monitoring the ECM for an increase in vacuum. If the control module detects that EVAP system vacuum increases above a calibrated value, DTC P0496 will set.
EVAP System Components
The evaporative emission (EVAP) system consists of the following components:
EVAP Canister
The canister is filled with carbon pellets used to absorb and store fuel vapours. Fuel vapour is stored in the canister until the control module determines that the vapour can be consumed in the normal combustion process.
EVAP Purge Solenoid
The evaporative emission (EVAP) purge solenoid controls the flow of vapours from the EVAP system to the intake manifold. This normally closed solenoid is pulse width modulated (PWM) by the control module to precisely control the flow of fuel vapour to the engine. The solenoid will also be opened during some portions of the EVAP testing, allowing engine vacuum to enter the EVAP system.
