Black spark plugs usually indicate which engine condition?

• A. Wrong spark plug voltage
• B. Engine is running lean
• C. Spark plug gap is incorrect
• D. Engine is running rich

Answer: (D)

Spark plug color is a diagnostic clue about how the engine is actually burning fuel. When a plug shows black, sooty deposits, it indicates carbon buildup from incomplete combustion due to an overly rich air-fuel mixture. With too much fuel relative to air, not all of it burns in the cylinder, leaving carbon on the plug tip and insulator. This carbon fouling points to the engine running rich, which is why that condition is concluded from the plug’s appearance. In contrast, a lean condition tends to produce lighter, tan to gray deposits and sometimes a hotter running plug, not the black soot seen with rich mixtures. A wrong voltage or a mis-sized gap can cause ignition issues, but they don’t produce the characteristic black soot associated with a rich fuel condition.