Evaporative Emissions System: Description and Operation

Evaporative Emission Control Schematic:

The evaporative emission 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 to retain 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:

VAPOR CANISTER
This is a basic two tube canister. Gasoline vapors from the fuel tank flow into the tube labeled "Fuel Tank". Any liquid fuel goes into a reservoir in the bottom of the canister to protect the carbon bed above. These vapors are absorbed into the carbon. The canister is purged when the engine is running above idle speed. A timed vacuum source is applied to the tube labeled "Canister Purge" to draw in fresh air through the bottom of the canister (open bottom). The air mixes with the vapor and is drawn into the intake manifold to be burned.


This system has thermostatic vacuum switch installed in the intake manifold coolant passage to sense engine coolant temperature. This thermal vacuum switch has two ports and is located between the canister and the TBI unit. When the engine is below 115 degrees FAHRENHEIT the thermal vacuum switch is closed preventing purge of the canister. When engine temperature is above 115 degrees FAHRENHEIT the thermal vacuum switch is opened allowing purge of the canister.