Which characteristic may cause preignition misfire, or fouling?

• A. Firing end
• B. Incorrect heat ranges
• C. Resistors
• D. Projected end

Answer: (B)

The key idea is how a spark plug’s heat range controls the tip temperature. If the plug’s heat range is incorrect, the tip can run either too hot or too cold. A too-hot plug overheats the tip, creating hot spots that can ignite the air-fuel mixture or any deposits before the spark fires, leading to preignition and misfire. A too-cold plug doesn’t get hot enough to burn off carbon and other deposits, causing fouling and a loss of reliable ignition. Because preignition and fouling are directly tied to how effectively the plug dissipates heat, incorrect heat ranges are the most likely causes among the options. The other features—firing end, resistors, and projected end—relate to design or electrical aspects of the plug but don’t by themselves determine the tendency to overheat or foul.