Evaporative Emissions System: Description and Operation

GENERAL DESCRIPTION - EVAP CANISTER PURGE 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 manifold vacuum, into the intake manifold where they are burned in the combustion process. The solenoid is controlled by the PCM using a pulse width modulated (PCM signal).

The canister purge solenoid is energized after engine start-up when:

^ Not at idle

^ Engine Coolant Temperature (ECT) above 56°C (133°F)

^ Throttle position above 2% open

^ Closed loop fuel control

^ When the non-purge long term fuel trim cells are not being updated

^ Non-purge long term fuel trim (LTFT) cells have learned. The PCM must be operating in a LTFT cell for approximately two minutes before it is indicated as learned.

The amount of duty cycle commanded to the purge solenoid will increase as manifold pressure increases.

Once the above conditions are met the fuel control system will switch over to the purge LTFT cells and the engine fueling requirements are then indicated by these cells. These cells, therefore, take into account fuel vapors.

When the start up coolant temperature is below 45°C (113°F) the learned fuel indicators are reset and the purge cells are updated to their corresponding non-purge cell (e.g., the purge LTFT cruise cell will be set equal to the non-cruise LTFT cell value).

EVAP CANISTER PURGE SYSTEM

The EVAP canister purge system is used to lower HC (hydrocarbon) emission levels by controlling the flow of fuel vapors into the engine, where they are burned along with the normal 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.