Repair Pontiac Mid-size 1974-1983 Repair Guide

Evaporative Emission Control System



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Fig. Fig. 1 The open canister Evaporative Emission Control System (EECS), shown here, is more common than the closed system

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Fig. Fig. 2 Closed canister Evaporative Emission Control System (EECS) schematic

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Fig. Fig. 3 The EVAP canister has several vacuum lines (see arrows) running from it to various components

This system reduces the amount of escaping gasoline vapors. 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 canister 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 and 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.

On 1981 and later V6s, the purge function 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 deenergized, thus allowing existing vacuum to operate the purge valve. This releases the trapped fuel vapor and it is forced into the induction system.

Most carbon canisters used are of the "Open" design, meaning that air is drawn in through the bottom (filter) of the canister. Some 1981 and later V6 canisters are of the "Closed" design, which means that the incoming air is drawn directly from the air cleaner.


The only service required for the evaporative emissions system is the periodic replacement of the charcoal canister filter. This procedure is covered in . If the fuel tank cap on your car ever requires replacement, make sure that it is of the same type as the original.