Evaporative Emissions System: Description and Operation

EVAP Control System Operation Description:

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

EVAP Canister:

Fuel Vapor Canister
Description
The Evaporative Emission (EVAP) control system uses a 1850 cc charcoal canister to absorb fuel vapors from the gas tanks.

Operation
When several conditions are met, the PCM will command the EVAP solenoid on and the vapor will flow to the canister where it is absorbed and stored by the charcoal. This allows the vapor to flow into the intake manifold for combustion.

Canister Purge Control Solenoid
Description
This system has a remote mounted canister purge control solenoid valve. The PCM operates this solenoid valve to control vacuum to the canister.

Operation
Under cold engine or idle conditions, the solenoid valve is closed, which prevents vacuum from being applied to the canister. The PCM activates (or opens) the solenoid valve and allows purge under the following conditions:
^ When the engine is started and coolant temperature is less than 50°C (122°F), purge will be delayed for 150 seconds.
^ When the engine is started and coolant temperature is greater than 50°C (122°F), purge will be delayed for 45 seconds.
^ Throttle angle greater than 3% to enable purge. Then, if the throttle angle drops below 2%, the purge will be disabled.
^ Engine air flow greater than 12 grams per second.
^ Vehicle speed greater than 16 km/h (10 mph) to enable purge.
Then, If the vehicle speed drops below 5.0 km/h (3 mph), the purge will be disabled.

RESULTS OF INCORRECT OPERATION
^ Poor idle, stalling and poor driveability can be caused by the following:
- Inoperative purge solenoid valve.
- Damaged canister.
- Hoses split, cracked and, or not connected to the proper tubes.
^ Evidence of fuel loss or fuel vapor odor can be caused by:
- Liquid fuel leaking from fuel lines.
- Cracked or damaged canister.
- Inoperative canister control valve.
- Vacuum hoses that are:
- Disconnected.
- Mis-routed-routed.
- Kinked.
- Deteriorated or damaged vapor hoses. If the solenoid valve is open, or is not receiving power, the canister can purge to the intake manifold at the incorrect time. This can allow extra fuel during warm-up, which can cause rough or unstable idle.