OBJECTIVE AND SCOPE The objective of this program was to investigate experimentally the effects of gasoline additives on carburetor and PCV system performance as they relate to exhaust emissions. Three groups of sixteen vehicles@ with each group using a different fuel-additive combination@ made up the forty-eight vehicle test population. All but three vehicles were driven for at least 12@000 urban and suburban miles under controlled fuel and maintenance conditions. During the accumulation of mileage@ engine tune-up maintenance was conducted in accordance with manufacturer specifications@ except for PCV valves and carburetor idle adjustments which were left untouched. Exhaust emissions were measured at the beginning of the program and at each subsequent 4@000-mile interval. Fourteen exhaust emission variables@ engine idle RPM@ and PCV valve flow were analyzed using the analysis of variance technique to identify significant differences among fuels@ mileage intervals@ vehicle make@ and the interactions among these factors. The duration and scope of the program were contractually curtailed during August@ 1970; consequently@ only a portion of the test vehicle population accumulated more than 12@000 miles. Data from tests made on vehicles whose test mileages exceeded 12@000 miles were utilized in regression analyses.