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 and sooty spark plugs indicate a rich fuel mixture, where there is more fuel than air and combustion isn’t fully burning the fuel. When the engine runs rich, excess fuel doesn’t combust completely, leaving carbon deposits on the electrode and insulator that build up as black soot. That carbon fouling is a hallmark of a rich condition, so the true statement is that the engine is running rich.

If the engine were running lean, plugs would tend to be dry and pale gray or white due to insufficient fuel to form carbon deposits. Using the wrong plug length affects heat range and sealing but not this soot pattern, while incorrect plug voltage mainly causes ignition problems or misfires rather than persistent carbon fouling.