Evaporative Emissions System: Description and Operation

Charcoal Canister:

SYSTEM OPERATION

The Evaporative Emission Control System (EECS) uses the basic charcoal canister storage method. This method transfers fuel vapor from the fuel tank to the charcoal canister storage device to hold the vapors when the vehicle is not operating. When the engine is running, the fuel vapor is purged form the carbon element by intake maniford air flow and is consumed in the normal combustion process.

CHARCOAL CANISTER

The EECS uses an inverted function charcoal canister. A double dip tube extends down the center of the canister to a liquid trap at the canister base. Fuel vapor from the tank enters a port at the top of the canister. Vapor (and any liquid droplets) travels down the dip tube between the inner dip tube and the outer tube wall. Any liquid present is trapped in the lower liquid well, away from the charcoal element. Fuel vapor is stored in the activated carbon element until purged.

PURGE MODE

When a purge mode is commanded, stored fuel vapor is drawn into the inner dip tube through a calibrated orifice at the base of the tube. Any liquid trapped in the liquid well is also drawn into the inner dip tube through a smaller calibrated orifice. Fresh air is drawn into the canister at the top mounted vent. The ECM operates a solenoid valve which purges the charcoal canister with ported vacuum at the throttle body. Under cold engine, "OPEN LOOP", or idle conditions, the solenoid is de-energized by the ECM, which closes the purge solenoid and blocks vacuum to the canister. The canister will be enabled (ECM energizes the solenoid) when:

1. Coolant is above 70~C.

2. Closed loop has been achieved for at least 9 seconds.

3. The throttle switch is open.

4. Codes E013, E044 or E045 is present.

If the solenoid is open, or is not receiving power, the canister will not purge to the intake manifold. This will prevent the canister from purging fuel vapors and could result in a complaint of fuel odors coming from the engine compartment.

When the engine is off, vapor is prohibited from entering the canister by means of a remote mounted Tank Pressure Control Valve (TPCV). The TPCV has a relief feature that allows vapor to pass from the fuel tank to the canister when the presure in the fuel tank exceeds 5.9 kPa (.85 psi).

When the engine is running, a manifold vacuum signal to the TPCV opens the valve and allows full-time vapor passage from the fuel tank to the charcoal canister. A vacuum relief feature allows vapor to travel form the canister back to the fuel tank during cool down periods.