The evaporative emission control system is designed to control the emission of gasoline vapors into the atmosphere. On carbureted engines, the vapors rising from the gasoline in the fuel tank are vented into a separate condensing tank which is located above the fuel tank. There they condense and return to the fuel tank in liquid form when the engine is not running.
When the engine is running, the fuel vapors are sucked directly into the engine through the PCV valve and are burned along with the air/fuel mixture.
Any additional fuel vapors which are not handled by the condensing tank are stored in a charcoal canister. When the engine is running, the charcoal is purged of its stored fuel vapor. On some models, a check valve vents the fuel vapor into the atmosphere if pressure becomes too excessive in the fuel tank.
The system on the carbureted models consists of a charcoal canister, check and cut valve, purge control valves, evaporator shutter valve in air cleaner.
On the pickup and MPV with fuel injection, the system takes fuel vapor that is generated in the fuel tank and stores it in the charcoal canister when the engine is not running. This fuel vapor remains in the canister until the engine is started at which time the fuel vapor is drawn into the dynamic chamber and burned. The system on these models is made up of the charcoal canister, purge control solenoid valves, a three-way check valve, vacuum switch control valve and an electronic control unit.
On the Navajo, the Evaporative Emission Control System provides a sealed fuel system with the capability to store and condense fuel vapors. The system has three parts: a fill control vent system; a vapor vent and storage system; and a pressure and vacuum relief system (special fill cap). The fill control vent system is a modification to the fuel tank. It uses an air space within the tank which is 10–12% of the tank's volume. The air space is sufficient to provide for the thermal expansion of the fuel. The space also serves as part of the in-tank vapor vent system. The in-tank vent system consists of the air space previously described and a vapor separator assembly. The separator assembly is mounted to the top of the fuel tank and is secured by a cam-lockring, similar to the one which secures the fuel sending unit. Foam material fills the vapor separator assembly. The foam material separates raw fuel and vapors, thus retarding the entrance of fuel into the vapor line.