FAA Powerplant Written Practice Test

What indicates that a spark plug is fouled?

1. It is discolored

2. It sparks without grounding

3. Its spark grounds without jumping electrodes

4. It shows physical damage

The correct answer is: Its spark grounds without jumping electrodes

A spark plug is considered fouled when it fails to perform its function properly, and one significant indicator of this is when the spark cannot jump the gap between the electrodes. In a functioning spark plug, the electrical energy should be able to create a spark that jumps across the gap, igniting the air-fuel mixture in the combustion chamber. When the spark plug is fouled, often due to carbon buildup, oil residue, or lead deposits, it cannot generate a sufficient spark, leading to a situation where the spark appears to ground out without jumping the electrodes. This is a clear sign that the spark plug is not working as intended and needs to be inspected or replaced. The other options provide indicators of different problems. Discoloration may suggest overheating or can be a sign of improper fuel mixtures; however, it does not specifically indicate fouling. A spark that ignites without grounding may occur in other contexts, and physical damage might suggest a different issue altogether, not strictly related to fouling. Therefore, the inability of the spark to jump the electrodes is a specific and reliable indicator of fouling in spark plugs.