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. On Turbo equipped vehicles a second purge control valve is used. This allows the canister to be purged by vacuum or by boost pressure while the turbocharger is in operation.

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 Vacuum Schematic:

PURGE CONTROL SYSTEM - TURBO EQUIPPED MODELS
On the turbo equipped model the Purge Solenoid is spring loaded open and closes when grounded by the MPI Control Unit. While the Purge Solenoid is open (ungrounded), manifold vacuum is bled to a port at the air-intake. When the MPI Control Unit grounds the Purge Solenoid, manifold vacuum is redirected to the lower vacuum chamber of the Dual Control Purge Valve. Vacuum pulls the valve open and the canister is purged by the depression within the air cleaner. Air cleaner depression is a form of vacuum, but much less than manifold vacuum. It is used on the Turbo Purge systems to allow the canister to be purged under a vacuum or a boost condition. The valve can be "pulled" open by a vacuum or "pushed" open by boost pressure.
When the Turbocharger is in operation, the bleed port at the air-intake becomes a boost feed nipple. If the Purge Solenoid is grounded (closed), boost pressure is directed to the pressure chamber of the Dual Control Purge Valve. This may cause the Purge Valve to open, allowing the canister to be purged.
When the Purge Solenoid is open (ungrounded), the ports of the Purge Solenoid are connected, causing equal boost pressures to be applied to both nipples of the Dual Control Purge Valve. This causes the spring to push the diaphragm and the valve closed, shutting off the canister purge. The same thing happens at Wide Open Throttle or heavy loads.
The MPI Control Unit will ground the purge control solenoid after the coolant sensor indicates higher than 158°F and the Air Flow Sensor indicates an intake air flow rate equivalent to 3,000 rpm.

Purge Control System Vacuum Schematic:

NON-TURBO EQUIPPED MODELS
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.