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Comparison of before-repair and after-repair test results for approximately 800 1981 and later model cars and light trucks recruited from customer service shows that the primary cause of excessive emissions depends on fuel metering technology (i.e., carburetor versus fuel-injection). With carbureted vehicles, mechanical component failure is the largest contributor to excessive emissions. Specifically, the need for adjustment or other repair of the carburetor is the single greatest cause of excessive emissions for carburetor-equipped vehicles. Ignition system maintenance and oxygen sensor replacement are the next most significant items. Electrical component failure is the largest source of excessive emissions for fuel-injected vehicles. Oxygen sensor failure is the single greatest source of excessive emissions and ignition system problems are second largest emissions source.

Evaporative hydrocarbon emissions from gasoline-powered vehicles continue to be a major concern in areas where the national ambient air quality standard for ozone is violated. As a result, accurate estimates of real-world emissions from in-use motor vehicles are of vital importance in assessing the progress made in reducing emissions, as well as in determining the need for and required magnitude of additional emissions reductions. In this study, real-world evaporative emissions testing was performed on 50 late-model vehicles (30 passenger cars and 20 light-duty trucks), ranging in age from the 1992 to 1997 model year. Six of the 50 vehicles were equipped with enhanced evaporative emission control systems. Forty-nine of the 50 vehicles were procured from an Arizona State Inspection and Maintenance (I/M) Program test lane located in Mesa, Arizona, and one vehicle was procured from an employee of the test facility.

Laboratory and inspection lane tests of a variety of 1981 and later model year passenger cars and light-duty trucks indicate that the test procedures currently specified for IM240 and Acceleration Simulation Mode (ASM) testing in vehicle inspection and maintenance (I/M) programs provide inadequate preconditioning for many vehicles. Changes to both the length of time and the speed-time profile of the preconditioning period are required to minimize false failures while minimizing the time required for testing. Under the current IM240 procedures, test results from the first 93 seconds of the test, called “Phase 1,” are ignored when the composite test results exceed the applicable standards. Phase 1 operation is presumed to be adequate for preconditioning and the pass/fail decision is made based on the last 146 seconds of the test, which is called “Phase 2.”