Evaporative Emissions System: Description and Operation

Evaporative Emission (EVAP) Control System:

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

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

Only when the following conditions are satisfied, the Engine Control Module (ECM) will energize the EVAP canister purge valve, allowing ported vacuum to be applied through the EVAP canister purge valve to the EVAP canister, thus purging the EVAP canister.

1. Engine running.
2. Engine Coolant Temperature (ECT) is high (engine at operating temperature).
3. Throttle valve opening not in the idle position.

As a result, fuel vapor in the EVAP canister is sucked 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.

Ported vacuum is applied to the EVAP canister through the EVAP canister purge valve when the ECM commands the EVAP canister purge valve "ON." When the ECT is low, the ECM interrupts the ground path to the EVAP canister purge valve. Therefore, ported vacuum is not applied to the EVAP canister In this state, the EVAP canister is not purged.

The TPC 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. 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.

FUEL LOSS OR VAPOR ODOR
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.

PURGE VALVE ALWAYS OPEN
If the EVAP canister purge valve is always energized (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.

PURGE VALVE ALWAYS CLOSED
If the EVAP canister purge valve is never energized (closed), the EVAP canister can become overloaded resulting in odor.