Evaporative Emissions System: Description and Operation

EVAPORATIVE (EVAP) EMISSION SYSTEM

EVAP EMISSION CONTROL SYSTEM PURPOSE
The basic evaporative emission (EVAP) control system used on all vehicles is the charcoal canister storage method. Gasoline vapors from the fuel tank flow into the canister through the inlet labeled "TANK." These vapors are absorbed into the activated carbon (charcoal) storage device (canister) in order to hold the vapors when the vehicle is not operating. The canister is purged by PCM control when the engine coolant temperature is over 60°C (140°F), the IAT reading is over 10°C (50°F), and the engine has been running. Air is drawn into the canister through the air inlet grid, The air mixes with the vapor and the mixture is drawn into the intake manifold.

EVAP EMISSION CONTROL SYSTEM OPERATION
The EVAP canister purge is controlled by a solenoid valve that allows the manifold vacuum to purge the canister. The powertrain control module (PCM) supplies a ground to energize the solenoid valve (purge on). The EVAP purge solenoid control is pulse-width modulated (PWM) (turned on and off several times a second). The duty cycle (pulse width) is determined by engine operating conditions including load, throttle position, coolant temperature and ambient temperature. The duty cycle is calculated by the PCM. The output is commanded when the appropriate conditions have been met. These conditions are:
^ The engine is fully warmed up.
^ The engine has been running for a specified time.
^ The IAT reading is above 10°C (50°F).
The EVAP purge vacuum switch is a normally closed switch positioned in the purge line between the canister and the EVAP purge solenoid. The EVAP purge vacuum switch will open when the vacuum increases to greater than 5 inches of water in the purge line. The EVAP purge vacuum switch is used by the PCM to monitor the EVAP canister purge solenoid operation and purge system integrity. The EVAP purge vacuum switch should be closed to ground with no vacuum present (0% EVAP purge PWM). With EVAP purge PWM at 25% or greater, the EVAP purge vacuum switch should open.
An incorrect EVAP purge system flow will set a DTC P0441. A continuous purge condition with no purge commanded by the PCM will set a DTC P1441. A fault in the EVAP purge vacuum switch circuit will set a DTC P1442.
Poor idle, stalling and poor driveability can be caused by:
^ A malfunctioning purge solenoid.
^ A damaged canister.
^ Hoses that are split, cracked, or not connected properly.

Legend
1. Throttle Body
2. EVAP Purge Control Solenoid
3. EVAP Canister
4. In-Tank Fuel Vapor Pressure Control Valve
5. Fuel Tank
6. Pressure Vacuum Vented Cap
7. Vapor Restriction