Evaporative Emissions System: Description and Operation

Description:

DESCRIPTION
The basic Evaporative Emission (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 air flow and consumed in the normal combustion process.

An Evaporative Emission (EVAP) control system is used to prevent emission of fuel vapor.

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

Only when the following conditions are satisfied does the Powertrain Control Module (PCM) open the vacuum passage of the EVAP canister purge valve.

^ Engine Coolant Temperature (ECT) sensor is high (engine at operating temperature).
^ Engine speed is higher than specified value.
^ Engine is running in Closed Loop.
^ Engine running within specified load.

As a result, fuel vapor in the EVAP canister is drawn into the intake manifold through the EVAP canister purge valve and purge hose. In this state, the EVAP canister is purged, or cleaned, by air drawn through the filter at the bottom of the EVAP canister.

TANK PRESSURE CONTROL VALVE
The Tank Pressure Control valve is used to keep the pressure in the fuel tank constant. When the pressure in the fuel tank becomes positive and reaches its specified value, the Tank Pressure Control valve allows the fuel vapor to flow into the EVAP canister. When the pressure in the fuel tank becomes negative and reaches its specified value, it allows air to 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.
- A malfunctioning purge solenoid.

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

- Liquid fuel leaking from fuel pipes.
- Cracked or damaged EVAP canister.
- Disconnected, misrouted, kinked, deteriorated or damaged vapor hoses.
- A broken or leaking solenoid and components.

EVAP PURGE VALVE OPEN
If the EVAP canister purge valve 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 a rough or unstable idle and an over rich condition.

EVAP PURGE VALVE CLOSED
If the EVAP canister purge valve is closed, the EVAP canister can become overloaded resulting in a noticeable fuel vapor odor complaint.