Evaporative Emissions System: Description and Operation

Evaporative Emission Control System:

The evaporative emission control system (EECS) is the charcoal canister storage type. Vapors from the fuel tank are stored in the canister when the engine is not running. When the engine is running, vapors are purged from the canister by intake air flow and consumed in the combustion process.

Fuel Vapor Canister:

VAPOR CANISTER

Fuel vapors flow from the tank to the canister and are stored in activated carbon (charcoal). The canister is purged when the engine is running above idle speed and coolant is above a specified temperature.


THERMOSTATIC VACUUM SWITCH (TVS)

The TVS is installed in the intake manifold coolant passage to sense temperature. The two TVS ports are connected to the vacuum hose between the canister and TBI unit. When engine temperature is below 46~C (115~F), the TVS is closed and prevents canister purge. The TVS opens above 46~C (115~F), allowing canister purge.


IN-TANK PRESSURE CONTROL VALVE

The in-tank pressure control valve, a combination rollover, integral pressure and vacuum relief valve, is located with the fuel sending unit in the fuel tank. When vapor pressure in the tank exceeds 5 kPa (1 inHg) the valve opens to allow vapor venting to the canister. When tank pressure is below the specified value, the valve closes and vapors are retained in the tank. The valve prevents vacuum build up in the fuel tank and rollover protection to prevent liquid fuel from entering the canister during normal driving maneuvers.