GM Full-Size Trucks 1970-1979 Repair Guide

Evaporation Control System


See Figure 1

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Fig. Fig. 1: Schematic of the evaporative emission control system

Introduced on California vehicles in 1970, and nationwide in 1971, 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 serve to reduce vapor loss into the atmosphere. The venting of fuel tank vapors into the air has been stopped. Fuel vapors are now directed through lines to a canister containing an activated charcoal filter. Unburned vapors are trapped here until the engine is started. When the engine is running, the canister is purged by air drawn in by manifold vacuum. The air and fuel vapors are directed into the engine to be burned.


Replace the filter in the engine compartment canister as specified in the Maintenance Intervals Chart in Routine Maintenance . If the fuel tank cap requires replacement, ensure that the new cap is the correct part for your truck.