Evaporative Emissions System: Description and Operation

Evaporative Emission Control System:

Evaporative Emission Canister:

Fuel Pressure Control Valve:

EVAPORATIVE EMISSION SYSTEM
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.

This system uses an evaporative emission purge control valve. This valve has 3 ports and is located between the canister and the TBI unit. When manifold vacuum is present the valve is open enabling purge, at high Manifold Absolute Pressure (MAP) or engine "OFF" conditions the valve is closed, thereby allowing the canister to absorb fuel vapors. This stops vapors from accumulating in the induction system.

EVAPORATIVE EMISSION CANISTER
Gasoline vapors from the fuel tank flow into the tube labeled tank. These vapors are absorbed into the carbon. The canister is purged progressively with throttle opening when the engine is running. 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.

TANK PRESSURE CONTROL VALVE
Normally vacuum and pressure relief are provided via the vapor pipe and canister. If abnormally high fuel tank pressure or vacuum occurs, the fuel tank gas cap is designed to relieve these conditions.