Evaporative Emissions System: Description and Operation

Evaporative Emissions Hose Routing:

CONTROL OF FUEL VAPORS
The evaporative emission control system (EECS) limits fuel vapor escape into the atmosphere. The system transfers fuel vapor from a sealed fuel tank, through a single vapor pipe, to an activated carbon (charcoal) storage device (EVAP vapor canister) that stores the vapors when the vehicle is not operating. When the engine is running with coolant temperature above 70°C (158°F), the vehicle speed above one mile per hour, and more than four percent TPS, fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.

Fuel vapors from the fuel tank are purged and flow into the EVAP vapor canister tube labeled fuel tank and are absorbed by the carbon. The EVAP canister is purged when the engine is running above idle speed. A timed vacuum source is applied to the EVAP vapor canister tube labeled EVAP canister purge to draw fresh air through the air inlet, at the top of the EVAP canister. This air flows through a tube to the bottom of the EVAP canister and forces the vapors out of the EVAP canister purge line.

EVAP CANISTER CONTROL
The PCM controls the EVAP canister purge system by energizing and de-energizing the EVAP canister purge solenoid by supplying a ground for the solenoid. When the solenoid is energized, fuel vapors are drawn, by timed vacuum, into the intake manifold where they are burned in the combustion process.

The fuel system used is of the non-venting type. This means that the fuel vapors that build-up inside the fuel tank are not released into the atmosphere. Instead they are directed through a hose to a charcoal canister located in the engine compartment. These fuel vapors are stored within the charcoal canister until the PCM decides that engine conditions are acceptable for the vapors to be drawn from the charcoal canister into the combustion chambers. The PCM uses a normally closed EVAP canister purge solenoid to control these vapors.

With the EVAP canister purge solenoid not energized, a valve within the solenoid body is closed and fuel vapors remain trapped within the charcoal canister. When the PCM supplies a ground for the solenoid, the solenoid energizes and opens the valve. This allows fuel vapors to be drawn by engine vacuum from the charcoal canister into the intake manifold where it will be burned inside the cylinders along with the normal combustion process.