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A three-year investigation was conducted to determine the cause of oil filter plugging in diesel engines. Laboratory, engine, and field tests showed that filter life does not correlate with filter media areas having highly dispersant oils. The oil holds several pounds of contaminant which is finely suspended until dump-out or agglomeration begins. Once agglomeration starts, it proceeds at a high rate, resulting in rapid filter plugging. Agglomeration or contaminant particle growth in oil depends on soot content, pH, level of oxidation products, presence of coolant or moisture, and oil additive chemistry. Filter analysis can be useful in solving field problems; in some cases, such analysis provides more information than oil analysis since the filter has stripped oil of problem evidence. Filters plug by six mechanisms (impaired dispersancy, excessive oil contamination, absorption/filtration of oxidation products, additive precipitation, gel formation, and wear debris). Water and coolant react with oil to precipitate additives, form gels or emulsions, and destroy oil dispersancy. Oils vary greatly in their water tolerance, and oil sump temperatures in winter during idle or stop-and-go service range from 120 to 170 degrees F. Blow-by condensate from a four-cycle diesel engine is extremely acidic, having pH values as low as 0.4. Small amounts of coolant in oil are difficult to detect with existing methods. Coolant evaporates, is removed by filters, forms gels with oil additives, and undergoes other reactions in the crankcase. Sodium level of filter paper ash is the most reliable means of detecting coolant leakage. Engine maintenance, not filter capacity, is the limiting factor on oil drain intervals. Filter plugging is a symptom of an engine or oil problem. Precipitated oil additive material imbedded in bearing overlay may cause overlay removal and make the bearing more susceptible to wear and corrosion. American Petroleum Institute classifications for diesel lubricants are inadequate because they have no provision for testing water tolerance nor the ability to measure wear and filter plugging with extended drains under winter operating conditions.