Evaporative Emissions System: Description and Operation

SYSTEM OPERATION

The evaporative emission system employed on this vehicle is of the charcoal canister design. This system is designed to prevent gasoline vapor emissions from escaping from the tank into the atmosphere. When the vehicle is not running, fuel in the fuel tank evaporates, and 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.

Solenoid Trio:

CANISTER PURGE SOLENOID

This solenoid is controlled by the Single Board Engine Controller (SBEC). The SBEC controls this solenoid by providing or removing the ground. When the engine temperature is below 27°C (80°F) the SBEC grounds the solenoid energizing it. Once 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 throttle body. When the engine reaches 27°C (80°F), and the specified time delay (100 seconds) has elapsed, the SBEC removes the ground from the solenoid, which de-energizes 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

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. 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 to enter the canister through the air tube in the top. The air mixes with the vapor and 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 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, and its primary purpose is to seal off the fuel tank in the event of accidental 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.