Evaporative Emissions System: Description and Operation

Fig. 70 Evaporative Emission Control System:

PURPOSE
The evaporative emission control (EVAP) system prevents evaporated fuel from the fuel tank and carburetor float bowl from entering the atmosphere.

OPERATION
When the engine is not running fuel vapors are vented from the fuel tank and carburetor float bowl into a canister containing activated charcoal.
- The fuel is stored in the charcoal until the engine is running and operating conditions are correct for the stored fuel to be purged.
- When fresh air is drawn into the canister through a filter located either at the bottom of the canister or in the air cleaner snorkel. The stored fuel is carried by the fresh air into the engine where it is burned in the combustion process.

FUEL TANK
- A specially designed domed fuel tank is used with the EEC system which allows for expansion of fuel and collection of fuel vapors.
- A fill pipe which prevents complete filling of the tank is used on these specially design tanks.
Liquid vapor separator prevents liquid fuel from entering the system.
- Liquid fuel entering the separator is spilled back into the tank while raw fuel vapors are passed into the lines.

FILLER CAP
The filler cap used on the EEC System is not vented under normal conditions.
- The filler cap contains a two-way relief valve to protect the tank from damage due to excessive pressure or vacuum within the tank, by venting the tank to atmosphere.

CAUTION: Be sure to use the proper filler cap for the application.
- Installation of a vented (non-emission) filler cap will render the system inoperative--since the system must be sealed to function as designed.
- Installation of a non-vented filler cap on a conventional tank, will result in a serious deformation or a total collapse of the fuel tank.