Evaporative Emissions System: Description and Operation

Evaporative Emission (EVAP) Control System:

PURPOSE
The purpose of the Evaporative Emission (EVAP) control system is to reduce Evaporative Hydrocarbons (HC). The basic EVAP control system used on all vehicles is the EVAP canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon (charcoal) storage device (EVAP 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 airflow and consumed in the normal combustion process.

OPERATION
The vapor generated in the fuel tank while driving, or with the engine at a stop, enters the EVAP canister where charcoal absorbs and stores the fuel vapor.

Only when the following condition is satisfied, is ported vacuum applied to the EVAP canister:

^ Engine coolant temperature is high enough to open the Evaporative Emission Thermal Vacuum Valve (EVAP TVV).

As a result, fuel vapor in the EVAP canister is sucked into intake manifold and burned in the combustion chambers. During this process, the EVAP canister is purged or cleaned by air drawn through the filter at the bottom of the EVAP canister.

Fuel Cap Pressure Control Valve:

The fuel cap pressure control valve is provided to keep pressure in the fuel tank constant. When the pressure in the fuel tank becomes positive and reaches its specified value, it opens the valve to let the vapor flow into the EVAP canister. On the other hand, when the pressure in the fuel tank becomes negative and reaches its specified value, it opens the valve to let-the air flow into the fuel tank.

RESULTS OF INCORRECT OPERATION
Poor idle, stalling and poor driveability can be caused by:
^ Damaged Evaporative Emission (EVAP) 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, or fuel pump.
^ Cracked or damaged EVAP canister.
^ Disconnected, misrouted, kinked, deteriorated or damaged vapor hoses, or control hoses.

THERMAL VACUUM VALVE
If the EVAP TVV is always open, the EVAP canister can purge to the intake manifold at all times. This can allow extra fuel at idle or during warm-up, which can cause rough or unstable idle, or too rich operation.

If EVAP TVV remains closed at all times, the EVAP canister can become overloaded resulting in odor.