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A phenomena that often occurs on the spark plugs for the Zetec engine is corona stain. People often mistake corona stain for leakage. Corona stain is a natural phenomenon that is caused by electrical stress in the air around the spark plug. The electrical stress attracts dirt in the air and this sticks to the spark plug. Spark plugs with a big gap need more voltage to make a spark so the electrical stress is bigger and there will be a bigger chance on corona stain. Corona stain is brown or brown-grey. Corona stain is NOT leakage of the spark plug and it has no negative effect on performance. (Leakage would only be possible in combination with overtightening and leaves black marks) |