GM Firebird 1967-1981 Repair Guide

Evaporative Emission Control

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OPERATION



See Figures 1, 2 and 3

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Fig. Fig. 1: Schematic of the open evaporative emissions system



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Fig. Fig. 2: Schematic of the closed evaporative emissions system



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Fig. Fig. 3: The EVAP canister (arrow) is located in one of the front corners of the engine compartment

This system, which was introduced on California cars in 1970 and other cars in 1971, reduces the amount of escaping gasoline vapors into the atmosphere. Float bowl emissions are controlled by internal carburetor modifications. Redesigned bowl vents, reduced bowl capacity, heat shields, and improved intake manifold-to-carburetor insulation reduce vapor loss into the atmosphere. The venting of fuel tank vapors into the air has been stopped by means of the carbon cannister storage method. This method transfers fuel vapors to an activated carbon storage device which absorbs and stores the vapor that is emitted from the engine's induction system while the engine is not running. When the engine is running, the stored vapor is purged from the carbon storage device by the intake air flow, then consumed in the normal combustion process. As the manifold vacuum reaches a certain point, it opens a purge control valve atop the charcoal storage canister. This allows air to be drawn into the canister, thus forcing the existing fuel vapors back into the engine to be burned normally.

In 1981, the purge function on some engines is electronically controlled by a purge solenoid in the line, which is itself controlled by the Electronic Control Module (ECM). When the system is in the "Open Loop" mode, the solenoid valve is energized, blocking all vacuum to the purge valve. When the system is in the "Closed Loop" mode, the solenoid is de-energized, thus allowing existing vacuum to operate the purge valve. This releases the trapped fuel vapor, which is forced into the induction system.

Most carbon canisters used on Firebirds are of the "Open" design, meaning that air is drawn in through the bottom (filter) of the cannister. Some canisters are of the "Closed" design which means that the incoming air is drawn directly from the air cleaner.

SERVICE



The only service required is the periodic replacement of the canister filter (if so equipped). This procedure is covered in Repair Guide . If the fuel tank cap on the Firebird ever requires replacement, make sure that it is of the same type as the original.

 
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