Evaporative Emissions System: Description and Operation

EVAP Control System Operation Description:

EVAP Canister:

DESCRIPTION
The Evaporative Emission (EVAP) Control system used on all vehicles is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon (charcoal) storage device (canister) to hold the vapors when the vehicle is not operating. When the engine is operating, the fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.

OPERATION
The EVAP purge solenoid valve allows manifold vacuum to purge the canister. The Powertrain Control Module (PCM) supplies a ground to energize the EVAP purge solenoid valve (purge on). The EVAP purge solenoid control is Pulse Width Modulated (PWM) or turned on and off several times a second.
The EVAP canister purge PWM duty cycle varies according to the operating conditions determined by the mass air flow, the fuel trim, the engine coolant temperature, and the intake air temperature. For certain [1][2]EVAP tests, the diagnostic will be disabled it the TP angle increases to above 75%.

COMPONENTS
The evaporative system includes the following:

^ Fuel tank
^ Evaporative emission canister vent solenoid
^ Fuel tank pressure sensor
^ Fuel pipes and hoses
^ Vapor lines
^ Fuel cap
^ Evaporative emission canister
^ Purge lines
^ Purge valve solenoid

DIAGNOSTIC SYSTEM MONITOR
The evaporative leak detection diagnostic strategy is based on applying vacuum to the EVAP system and monitoring for vacuum decay.

The fuel level sensor input to the PCM is used to determine if the fuel level in the tank is correct to run the [1][2]EVAP diagnostic tests. To ensure sufficient volume in the tank to begin the various diagnostic tests, the fuel level must be between 15% and 85%.

The PCM monitors the fuel tank pressure/vacuum level via the fuel tank pressure sensor input.

The following conditions must be present before the EVAP system diagnostic runs:

^ No TP sensor, 0DM, IAT sensor, or MAP sensor DTCs set.
^ Engine coolant temperature is between 4°C and 30°C (40°F and 86°F).
^ Start up engine coolant temperature is not more than 8°C (14°F) greater than start up intake air temperature.
^ Intake air temperature is between 4°C and 30°C (40°F and 86°F).
^ Start up intake air temperature is not more than 9°C (46°F) greater than start up engine coolant temperature.
^ Fuel tank level is between 15% and 85%
^ BARO is above 75 kPa.

RESULTS OF INCORRECT OPERATION

^ Poor idle, stalling and poor driveability can be caused by the following:

- Malfunctioning purge solenoid.
- Damaged canister.
- Hoses/lines split, cracked and/or not connected properly.

^ Evidence of fuel loss or fuel vapor odor can be caused by the following:

- Liquid fuel leaking from the fuel lines
- Cracked or damaged canister
- Inoperative canister control valve
- Vacuum hoses that are:
- Disconnected
- Mis-routed
- Kinked
- Deteriorated
- Damaged