Evaporative Emissions System: Description and Operation

Air Cleaner Venturi:

Canister Purge Operation:

This system is used to prevent fuel vapors from escaping into the atmosphere. This is accomplished through the use of a non-vented fuel tank, a special fuel filler cap, charcoal canister, rollover/pressure relief valve, purge shutoff switch, and hoses necessary to connect these components together.


CHARCOAL CANISTER
The fuel resistant nylon body of the canister contains activated charcoal granules which absorb and store fuel vapors until they are drawn into the induction system to be burned along with the normal air fuel mixture. The evaporative canister is equipped with a purge shut off switch that controls canister purge operation. The switch is open when manifold vacuum is applied to it. The air cleaner contains a venturi in the air cleaner assembly as a purge line vacuum source.
The effect of the venturi increases the speed of the intake air flowing by the slots in the venturi wall. This creates a low pressure area around the slots. When the purge shutoff switch is open, vapors from the canister are drawn through the slots and into the combustion chambers where they are consumed during combustion.





FUEL FILLER CAP
The fuel filler cap uses a two-way (vacuum and pressure) relief valve that is closed during normal operating conditions. The relief valve is calibrated to open only when pressure in excess of 1.5 psi or vacuum in excess of 1.8 inch Hg develops in the fuel tank. When vacuum or pressure is relieved, the valve returns to its normally closed position.
It is normal to occasionally encounter an air pressure release when removing the fuel filler cap.

Pressure Relief/Rollover Valve:

ROLLOVER CHECK VALVE
This valve prevents the flow of fuel through the fuel line should the vehicle be rolled over. The check valve consists of a plunger and a stainless steel ball. When inverted, the stainless steel ball pushes the plunger against its seat.