Which statement is true when spark plugs are black and sooty?

• A. Engine is running lean
• B. Wrong spark plug length
• C. Wrong spark plug voltage
• D. Engine is running rich

Answer: (D)

Black, sooty spark plugs show a rich air–fuel mixture in the cylinder. When there’s more fuel than air, not all of it burns, leaving carbon deposits on the spark plug tip and insulator. This fouling is a hallmark of running rich and can lead to slower combustion and misfires if it worsens. A lean condition would produce a lighter-colored deposit or a more gray/white appearance due to hotter combustion, not black soot. Using a spark plug with the wrong length or wrong voltage mainly affects spark quality and heat management rather than causing the distinctive carbon fouling pattern. So, the true statement is that the engine is running rich.