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As part of the Auto/Oil Air Quality Improvement Research Program (AQIRP), a fleet of 299 1983-1993 “real world” light duty vehicles and trucks were acquired from inspection and maintenance (I/M) lanes and tested at prevailing ambient temperatures for their hot soak emissions for the first hour after the engine was turned off. When found, high-emitters were repaired and retested to quantify the effectiveness of the repairs. Also, I/M pressure-purge tests were performed to determine whether such tests could properly identify high-emitting vehicles. Measured hot soak emissions ranged from less than 0.1g HC to as high as 49g HC. Twenty percent of the vehicles tested accounted for nearly 80 percent of the total hot soak emissions, with no single common hardware component identified as the primary cause.