Evaporative Emissions System: Description and Operation

EVAP Emission Control System Schematic:

PURPOSE
The Evaporative Emission (EVAP) control system used on this vehicle is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon storage canister. This retains the fuel 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.

OPERATION
The evaporative emission control system uses a rear mounted charcoal canister to absorb fuel vapors from the gas tank.
When gasoline vapor builds enough to overcome the spring tension of the EVAP Control Valve, the vapor will flow to the canister where it is absorbed and stored by the charcoal. Under certain operating conditions the PCM commands the purge solenoid to open, allowing vapor flow into the intake manifold for combustion.

RESULTS OF INCORRECT OPERATION
- Poor idle, stalling and poor driveability can be caused by:
- Inoperative EVAP canister purge valve.
- Damaged 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.
- Inoperative canister control valve.
- Cracked or damaged EVAP canister.
- Disconnected, misrouted, kinked, deteriorated or damaged vapor hoses, or control hoses.

If the solenoid valve is always open, or not receiving power, the 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 the EVAP canister purge valve is always closed, the canister can become over-loaded, resulting in fuel odor.