Evaporative Emissions System: Description and Operation

The evaporative emission system prevents gasoline vapors in the fuel system from escaping into the atmosphere. When the vehicle is not running fuel in the fuel tank evaporates, the vapors travel through vent hoses or tubes to be stored in the charcoal canister. Once the vehicle is running, these vapors are drawn into the engine and consumed with combustion gases.

The system consists of a Charcoal Canister Purge Solenoid, a Charcoal Canister, a Pressure Vacuum Filler Cap, and a Rollover and Pressure Relief Valve.

Canister Purge Solenoid Location:

CANISTER PURGE SOLENOID

The engine controller, controls the cannister purge solenoid by providing or removing the solenoid's ground. When engine temperature is below 27~C (80~F), the engine controller grounds the solenoid (energizing it). While the Purge Solenoid is grounded, no vacuum is allowed to flow to the charcoal canister control valve. If this valve has no vacuum, the canister is sealed and cannot vent to the engine. When engine temperature reaches 27~C (80~F), and the specified delay time has elapsed (100 seconds), the engine controller removes the ground from the solenoid, de-energizing it, allowing vacuum to be applied to the charcoal canister control valve. Once vacuum is applied to the valve, the charcoal canister vapors are then purged through the throttle body.

Charcoal Canister:

CHARCOAL CANISTER

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

Pressure-Vacuum Filler Cap:

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 a malfunction in the system or damage to the vent lines.

Rollover and Pressure/Relief Valve:

ROLLOVER AND PRESSURE RELIEF VALVE

This valve is located on top of the fuel tank, seals off the fuel tank in the event of vehicle rollover. The relief valve portion of the valve allows fuel vapors to vent to the charcoal canister. It also allows fuel tank pressure to vent to the atmosphere when the fuel tank pressure exceeds the calibrated sealing valve setting.