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Why Does Engine Oil Turn Black?

1 Answers

LiamMarie

07/25/25 2:39pm

Engine oil is a lubricant, but it also functions as a detergent. When the engine is running, the oil pump operates under pressure, circulating oil to various friction points like water through pipes. While providing lubrication, it also carries wear particles back to the oil pan. Over time, these deposits accumulate, causing the oil to gradually darken. Reasons for engine oil turning black: 1. High olefin and sulfur content in fuel: This accelerates oil darkening, making fuel quality a primary source of oil discoloration. 2. Differences in engine structure and operating conditions: Frequent start-stop driving patterns can easily lead to sludge formation in gasoline engines, while incomplete combustion in diesel engines produces soot deposits, both causing oil darkening. 3. Prolonged high-temperature operation under overload conditions: Sludge formation is also related to extended periods of high-temperature overload operation, failure to clean the engine and oil pan during oil changes, and the filtration efficiency of the oil filter.

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