Chevrolet Full-size Cars 1968-1978 Repair Guide

Evaporative Emission Control



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Fig. Fig. 1 Schematic of the early type evaporative emissions system

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Fig. Fig. 2 Schematic of the evaporative emissions system found on later models

This system which was introduced to California vehicles in 1970, and all other vehicles in 1971, 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 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.

Most carbon canisters used are of the `Open' design, meaning that air is drawn in through the bottom (filter) of the canister.


The only service required is the periodic replacement of the canister filer (if so equipped). If the fuel tank cap on you vehicle ever requires replacement, make sure that it is of the same type as the original.


  1. Loosen the screw holding the canister retaining bracket.
  3. Rotate the canister retaining bracket and remove the canister.
  5. Tag and disconnect the hoses leading from the canister.
  7. Installation is in the reverse order of removal.

  1. Remove the vapor canister.
  3. Pull the filter out from the bottom of the canister.
  5. Install a new filter and then replace the canister.