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Emission studies and component analyses were carried out on Clear-fueled and MMT®-fueled 100,000 mile Escort vehicles from the Alliance study [SAE 2002-01-2894]. Previously reported analyses of these vehicles indicated that all differences in emission system performance could be attributed, with a 90% confidence level, to the engine cylinder head, spark plugs, oxygen sensors, and catalysts [SAE 2004-01-1084]. These parts from the Clear and MMT®-fueled vehicles were further analyzed to determine the root causes of the differences in emission system performance. The intake/exhaust valves, fuel injectors, and EGR valves from the cylinder heads were tested, individually and in groups, for differences in vehicle emission performance. Deposits from the exhaust valves of the MMT®-fueled vehicle were characterized by X-ray diffraction (XRD) and energy-dispersive X-ray spectrometry (EDX), and shown to resemble Mn3O4 with partial substitution of Zn2+ for Mn2+.