Evaporative Emissions System: Description and Operation

The evaporative emission control system 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 to retain the 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.




CHARCOAL CANISTER

Gasoline vapors from the fuel tank flow into the tube labeled "Fuel Tank". Any liquid fuel goes into a reservoir in the bottom of the canister to protect the carbon bed above. These vapors are absorbed into the carbon. The canister is purged when the engine is running at idle and off idle by utilizing a dual source of vacuum. Ambient air is allowed into the canister through the air tube in the top. The air mixes with the vapor and is drawn into the intake manifold.



CANISTER PURGE SOLENOID

The canister purge solenoid is controlled by the Engine Controller. When engine temperature is below a predetermined level, and for a specified time after hot starts, the computer grounds the purge solenoid, energizing it. This prevents vacuum from reaching the charcoal canister control valve. When engine temperature reaches approximately 145°F, and the time delay has expired, the Engine Controller de-energizes the solenoid by turning the ground off. Once this occurs, vacuum will flow to the canister purge valve and purge fuel vapors through the throttle body.


PRESSURE-VACUUM FILLER CAP

The fuel tank is sealed with a specially engineered pressure vacuum relief filler cap. The relief valves in the cap are a safety feature, and operate only to prevent excessive pressure or vacuum in the tank caused by malfunction in the system or damage to the vent lines.




ROLLOVER & PRESSURE RELIEF VALVE

This valve provides a path for fuel vapors to travel from the fuel tank to the canister and provides fuel leakage protection when a vehicle rollover occurs. This valve also incorporates a pressure relief mechanism designed to release pressure to the atmosphere when fuel tank pressure exceeds that of the calibrated sealing valve.