GM Buick/Oldsmobile/Pontiac/Full-Size 1975-1990 Repair Guide

Evaporative Emission Control System


See Figures 1, 2, 3, 4 and 5


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 de-energized, thus allowing existing vacuum to operate the purge valve. This releases the trapped fuel vapor and it is forced into the induction system.

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Fig. Fig. 1: Canister Control Valve (CCV)

Some canister systems (those without a vapor vent valve) starting in 1981 have a Canister Control Valve (C.C.V.). This is mounted near the carburetor and has four hoses connected to it. When the engine is off, manifold vacuum is non-existent at the C.C.V. and a spring loaded valve in the C.C.V. interconnects the carburetor vent hose to the canister via a Thermostatic Vacuum Valve which opens at 170°F (77°C). Vapors generated in the carburetor float bowl thus pass into the canister. When the engine is restarted, this valve closes as manifold vacuum is applied to it. When the TVS is open, the canister is purged as fuel vapors are drawn out of the canister and into the carburetor throttle body.

Most carbon canisters used are of the open design, which means that the incoming air is drawn directly from the air cleaner.

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Fig. Fig. 2: Open canister evaporative emission control system (EECS). This system is more common than the closed system

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Fig. Fig. 3: Closed canister EECS

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Fig. Fig. 4: Common EECS canister

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Fig. Fig. 5: This model has a TVS located on the air cleaner


The only service required for the evaporative emissions system is the periodic replacement of the charcoal canister filter. Closed canisters do not require that this operation be performed. If the fuel tank cap on your vehicle ever requires replacement, make sure that it is of the same type as the original.


  1. Tag and disconnect all hoses.
  3. Loosen the retainer clamps and lift out the canister.
  5. Grasp the filter element and pull it out.
  7. Replace the filter, then place the canister in the clamps and reconnect all hoses.