Evaporative Emissions System: Description and Operation

Fig.29 - Evaporative Emission Control System:

The evaporative emission control system prevents the escape of gasoline vapors from fuel tank into atmosphere. Various systems differ in operation due to addition or change of various components to accomplish this more efficiently.

Starting with 1978, this system replaces idle cut-off valve with a purge control solenoid valve which, when ignition switch is turned on, opens canister purge line to carburetor venturi. When venturi vacuum reaches preset level, fuel vapor from canister is drawn through purge control solenoid valve to carburetor venturi. When ignition is turned off, urge control solenoid valve shuts down line tween canister and carburetor. 1979 Civic operation is the same as 1978.

On 1980 Civic, purge control solenoid valve is controlled by a thermosensor which reacts to engine coolant temperature. This system also incorporates an air vent cut-off diaphragm which, when engine is off, cuts off air vent passages in main and auxiliary float chambers, venting fuel vapors in float chambers into canister. Manifold vacuum holds diaphragm open when engine is running. A vacuum holding valve stabilizes manifold vacuum at diaphragm. A primary main fuel cut-off solenoid valve cuts off fuel passages for main and slow primary fuel metering system when engine is off so that fuel from float chamber cannot enter carburetor bore.

On 1981 Civic, a power valve control solenoid is added on Calif. models which, when ignition is off, is open to a check valve which holds vacuum between check valve and power valve. Power valve is then closed, cutting off fuel passage.

On 1982 Civic, a thermovalve "B" is added which supplements the purge control solenoid valve. The power valve control solenoid is incorporated on all models.