Evaporative Emissions System: Description and Operation

EVAP Emission Control System:

The evaporative emission control system is used to prevent emission of fuel vapors. The vapors generated in the fuel tank while driving or when the engine is at a stop enters the charcoal canister where the charcoal absorbs and stores the fuel vapor. Only when the following conditions are all satisfied is throttle body vacuum applied to the canister purge control valve to open the valve.

^ Engine coolant temperature is high (engine is warmed up).

With vacuum applied to the charcoal canister, fuel vapor in the canister is drawn into the intake manifold through the purge line. In this state, the canister is purged or cleaned by air drawn through the filter at the bottom of the canister. When engine coolant temperature is low, the Evaporative Thermal Vacuum Valve (EVAP TVV) is closed, preventing vacuum from reaching the canister.

When coolant temperature is low, the EVAP TVV valve is closed and vacuum is not applied to the charcoal canister. In this state, the canister is not purged.

Fuel Pressure Control Valve:

A fuel cap check valve is provided to keep the pressure in the fuel tank constant. When the pressure in the fuel tank becomes positive and reaches its specified value, the purge control valve opens to let the vapor flow into the charcoal canister. On the other hand, when the pressure in the fuel tank becomes negative and reaches its specified value, the fuel cap check valve opens to let 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.

If EVAP thermal vacuum valve (EVAP TVV) is 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 is closed the EVAP canister can become overloaded resulting in odor.