Evaporative Emission Control System Operation Description. Geo Prizm 1996

For 1990-2009 cars

### Purpose

The EVAP control system limits the fuel vapor emissions to the atmosphere. The EVAP control system transfers the fuel vapor from a sealed fuel tank to an activated carbon (charcoal) storage device (EVAP canister). The EVAP canister will store the fuel vapors until the engine is able to use them.

### EVAP Control System Operation

The fuel vapor generated in the fuel tank while driving or idling, passes through a tank pressure control valve and enters the EVAP canister. The EVAP canister contains a charcoal element that absorbs and stores the fuel vapor. When the engine is able to use the stored fuel vapor, the fuel vapor is purged from the carbon element by intake air flow (low intake manifold pressure) and consumed in the normal combustion process.

An EVAP canister purge vacuum switch is used to control the purging of fuel vapors stored in the EVAP canister. The EVAP canister purge vacuum switch is a temperature controlled vacuum valve. The EVAP purge vacuum switch is mounted on the intake manifold and opens when the engine coolant temperature is more than 53°C (127°F). When the EVAP purge vacuum switch is open, manifold vacuum is applied to the vapor space of the EVAP canister, drawing fuel vapors into the intake manifold. The fuel vapors are then drawn into the engine cylinders and consumed in the normal combustion process.