Evaporative Emissions System: Description and Operation

Evaporative Emission Control System Schematic:

PURPOSE
The Evaporative Emission Control System (EECS) used on this vehicle is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon storage canister. This retains the fuel vapors when the vehicle is not operating. When the engine is running, the fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.



OPERATION

Fuel Vapor Canister:

Fuel Vapor Canister
The evaporative emission control system uses an inversely mounted 1500 cc charcoal canister to absorb fuel vapors from the gas tank.


When gasoline vapor builds enough to overcome the spring tension of the Tank Pressure Control Valve (TPCV), the vapor will flow to the canister where it is absorbed and stored by the charcoal. Under certain operating conditions the PCM commands the purge solenoid to open, allowing vapor flow into the intake manifold for combustion.

EVAP Canister Location.:

Purge Solenoid Service:

Evaporative Control
The PCM operates the solenoid to control vacuum to the canister. Under cold engine or idle conditions, the solenoid is closed, which prevents vacuum from being applied to the canister.

The PCM activates the solenoid valve and allows purge under the following conditions:

^ Engine is warm
^ After the engine has been running a specified period of time
^ Above a specified road speed
^ Above a specified throttle opening

Tank Pressure Control Valve:

Tank Pressure Control Valve
When pressure in the tank exceeds approximately 5 kPa (.7 psi) the diaphragm valve opens, allowing vapors to vent to the canister. A 1.14 mm (0.045 in.) orifice in the passage leading to the canister tube causes pressure to drop slowly, preventing the valve from oscillating (buzzing). When the tank pressure drops below 5 kPa (.7 psi) the diaphragm valve closes, causing vapors to be held in the fuel tank.