I/M Short Tests and Cutpoints for 1981 and Newer Emission Control Technology

Manufacturers have used a number of alternative emission control technologies to meet the emission standards for 1981 and later model years. Each technology type can have malperformances that lead to high emissions, but the ability of inspection/maintenance short tests to identify high emitters varies considerably by technology. In this study, the relationships between malperformance types, FTP emissions and short test emissions were identified for several major technology types, using data from the EPA Emission Factor data base. Based on these relationships, methods to enhance the effectiveness of I/M tests are suggested. The analysis indicated that the most important factor governing the ability of any I/M short test is the presence of absence of secondary air. The short tests were found to be incapable of recognizing any malperformance except severe misfire if secondary air was present. Without secondary air, the short tests could identify many common malperformances leading to high emissions.