Evaporative Emission Control System Operation

The evaporative emission system prevents gasoline vapors in the fuel system from escaping into the atmosphere. When the vehicle is not running fuel in the fuel tank evaporates, the vapors travel through vent hoses or tubes to be stored in the charcoal canister. Once the vehicle is running, these vapors are drawn into the engine and consumed with combustion gases.

The system consists of a EVAP Purge Solenoid, a Charcoal Canister, a Pressure Vacuum Filler Cap, and a Rollover and Pressure Relief Valve.

EVAP PURGE SOLENOID
Vapor flow through the charcoal canister is controlled by the EVAP Purge solenoid. The EVAP Purge solenoid regulates vacuum to the purge shut off switch on the charcoal canister. The Powertrain Control Module (PCM) controls solenoid by providing an electrical ground path. When energized (solenoid grounded), vacuum is prevented from reaching the purge switch. When de-energized vacuum is applied to the purge switch. The purge switch, when vacuum is applied, allows vapors to be drawn into intake manifold where they are burned during combustion.

EVAP Canister (Typical) Operation:

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. Any liquid fuel goes into a reservoir in the bottom of the canister to protect the carbon bed above. The vapors are absorbed by the carbon. The canister is purged when pre-determined operating conditions have been met.

Pressure-Vacuum Filler Cap:

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.
NOTE: It is normal to occasionally encounter an air pressure release when removing the fuel filler cap.

Pressure Relief/Rollover Valve:

PRESSURE RELIEF/ROLLOVER VALVE
These vehicles are equipped with only one rollover/pressure relief valve. These dual function valves relieve fuel tank pressure and prevent fuel flow through the fuel tank vent lines should the vehicle be rolled over. The valve consists of a plunger, spring, and an orifice and guide plate. The valve is normally open. In the closed position, force applied by spring pressure seats the plunger against the guide plate, shutting off the flow of fuel vapor from the fuel tank to the evaporative canister. When fuel does not reach the valve, the spring pressure is overcome and the plunger moves away from the guide plate allowing fuel vapor to flow through the valve orifice.

In vehicle rollover the valve is inverted. In this position the plunger is forced against the guide plate and raw fuel is prevented from flowing through the valve orifice into the fuel tank vent tube.