Evaporative Emissions System: Description and Operation

This system is designed to prevent raw fuel vapors from escaping into the atmosphere by the use of one or more of the following components, a fuel tank with a sealed filler cap, fuel and vapor separator, check valves, charcoal canister, vacuum switching valve, speed sensor, computer, purge valve and a connection at either the intake manifold or carburetor.
The sealed filler cap incorporates a check valve which opens to allow outside air into the tank, when the pressure within the tank drops below a predetermined value. A defective filler can cause severe deformation of the fuel tank.