What does black, sooty spark plugs most commonly indicate about engine operation?

What does black, sooty spark plugs most commonly indicate about engine operation?

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

Answer: (C)

Black, sooty spark plugs show the engine is running rich—there’s more fuel than air being burned. When fuel is in excess, not all of it combusts cleanly and carbon deposits build up on the plug’s electrode and insulator, turning it dark and sooty. This soot can insulate the spark and worsen ignition and fuel economy. By contrast, a lean condition tends to produce light-colored or white deposits and can cause hot plugs or misfires. Issues like incorrect plug length or wrong voltage mainly affect heat range or spark quality rather than creating the characteristic black carbon buildup seen with a rich mixture.

Black, sooty spark plugs show the engine is running rich—there’s more fuel than air being burned. When fuel is in excess, not all of it combusts cleanly and carbon deposits build up on the plug’s electrode and insulator, turning it dark and sooty. This soot can insulate the spark and worsen ignition and fuel economy. By contrast, a lean condition tends to produce light-colored or white deposits and can cause hot plugs or misfires. Issues like incorrect plug length or wrong voltage mainly affect heat range or spark quality rather than creating the characteristic black carbon buildup seen with a rich mixture.