Evaporative Emissions System: Description and Operation

Evaporative Emission Control System:

The Evaporative Emission Control System is designed to prevent fuel vapors from being emitted into the atmosphere from the fuel tank. The system consists of a Pressure/Vacuum Relief Filler Cap, a Rollover Check Valve, a Charcoal Canister, and connecting lines and hoses. Canister purging is controlled by the MPI Control Unit through a Purge Control Solenoid Valve.

Charcoal Canister, Typical:

CHARCOAL CANISTER
The charcoal canister is located under the hood and is filled with activated charcoal granules, which will hold up to one-third their own weight in fuel vapors. Activated charcoal is used as a vapor trap because of its great surface area. Each gram of activated charcoal has a surface area of 1,100 square meters, or more than a quarter acre. The canister is connected to the fuel tank by a vent line. Within the vent line is a two-way check valve which opens to allow excess fuel tank pressure and vapors into the canister for storage, until the engine is started. During the canister purge cycle fresh air is drawn through the filter at the bottom of the canister, purging the charcoal of fuel vapors.

Fuel Filler Cap:

PRESSURE/VACUUM FILLER CAP
The pressure-vacuum filler cap is designed to hold a slight pressure on the fuel which occurs from normal heat expansion. The evaporation rate decreases under slightly higher pressures, so the cap helps to reduce evaporative emissions. When vacuum in the tank exceeds specifications, the vacuum relief portion of the filler cap will allow atmospheric pressure to enter the tank. If the pressure within the tank exceeds a specified value, the pressure relief valve will open to relieve excess pressure. This would occur when the pressure relief check valve within the fuel vapor line became plugged.

Check Valves:

ROLLOVER AND PRESSURE RELIEF CHECK VALVES
The rollover check valve is installed within the fuel line between the fuel tank and the fuel injectors. Its primary purpose is to seal off the fuel tank in the event of accidental vehicle rollover. The pressure relief check valve is located within the vapor line between the fuel tank and the charcoal canister. It is designed to vent excess pressure and fuel vapors from the tank into the charcoal canister, when the fuel tank pressure becomes positive and reaches a specified value.

Purge Control System Operation:

PURGE CONTROL SYSTEM
The Purge Control Solenoid Valve is spring loaded closed (blocks purge), and opens when grounded by the MPI Control Unit. The Purge Control Solenoid will be grounded by the MPI Control Unit after the engine coolant temperature sensor indicates 158°F or higher; and the Air Flow Sensor indicates an intake air flow rate equivalent to 3000 rpm. Once the valve opens, manifold vacuum will be applied to the canister, purging it of fuel vapors.