One evaporative emission control system diagram provides a visual description of the system from outside a vehicle, showing a typical arrangement of system components in relation to fuel tank components. Other diagrams compress the various system components but include more detail. System diagrams for specific vehicles are also available.
The evaporative emission control system or EVAP seals off the fuel system from the atmosphere by sending fuel vapors through vent lines to a storage container. Vapors are contained until the engine is started; then a purge valve allows them to be pulled into the intake manifold and burned in the engine. The diagram showing the system from an exterior perspective identifies the point at which the liquid and vapors are separated as they exit the fuel tank, and the diagram also displays the vent line through which the vapors are directed to the storage container. It shows the purge valve sitting atop the container and a purge line through which the vapors are siphoned into the carburetor or manifold.
More detailed diagrams compress the physical placement of the various system parts, marking the purge and vent valves (also called solenoids), their control outputs and the purge valve's connection to the car's computer. Monitoring for leaks requires the creation of a vacuum, and detailed diagrams usually include schematics for tank and engine vacuum sensors.