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This paper presents the results of a twenty- vehicle program designed to determine the functional relationship between intake valve deposit level and exhaust emissions. The “identical” 1990 model year vehicles used for this program had accumulated over 80,000 kilometers in taxi fleet service and had developed average intake valve deposit levels which ranged from 6 to 9 on the Coordinating Research Council merit scale. The exhaust emissions from these vehicles were measured in triplicate tests using the 1975 Federal Test Procedure. The intake valve deposits were then mechanically cleaned in-situ, and exhaust emissions were again measured in triplicate. Special procedures were followed to minimize vehicle-to- vehicle variability and to obtain statistically meaning ful results. Results showed intake valve deposits to have a significant adverse effect on exhaust emissions.