Evaporative Emissions System: Description and Operation

Evaporative Vacuum Diagram:

DESCRIPTION AND OPERATION
The basic Evaporative Emission (EVAP) Control System used on all Vehicles is the charcoal canister storage method. This method transfers Fuel vapor from the Fuel Tank to an activated carbon (charcoal) storage device (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.

VAPOR CANISTER
Gasoline vapors from the Fuel Tank flow into the tube labeled tank. Any liquid Fuel goes into a reservoir in the bottom of the canister to protect the integrity of the carbon bed above.




These vapors are absorbed into the carbon. The Canister is purged when the Engine is running or commanded by Engine Control Module (ECM). Ambient air is allowed into the Canister through the air tube in the top. The air mixes with the vapor and the mixture is drawn into the Intake Manifold.

EVAP CONTROL SYSTEM
The Solenoid used with this Canister uses Vacuum Switch Valve to control purge. The ECM opens and closes the Solenoid to control purge.

The ECM operates a Normally Closed (N/C) Solenoid Valve which controls the vacuum to purge the Charcoal Canister. Under cold Engine or idle conditions, the Solenoid is not energized by the ECM, which blocks vacuum to purge the Canister.
The ECM energizes the solenoid valve and allows purge when:
- Engine is warm above 69°C (156°F).
- After the Engine has been running a specified time.
- Throttle position is above 7% Throttle Position Densor.

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

If the Solenoid is always open, 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 Solenoid is always closed, the Canister can become over-loaded, resulting in Fuel odor.