FAA Aviation Mechanic Powerplant (AMP)Engine InspectionEasy
A student mechanic removes a spark plug from a reciprocating engine cylinder and notices the electrodes are coated with a heavy black, dry, fluffy deposit. What condition does this indicate?
- ACarbon fouling, often caused by an excessively rich fuel/air mixture or extended ground idling
- BNormal wear consistent with proper engine operation
- CLead fouling from excessive tetraethyl lead in the fuel
- DOil fouling from worn piston rings
Show answer & explanationAnswer & explanation
Correct answer: A. Carbon fouling, often caused by an excessively rich fuel/air mixture or extended ground idling
A dry, black, fluffy or sooty deposit on spark plug electrodes is characteristic of carbon fouling, typically caused by an overly rich mixture, prolonged idling, or insufficient combustion temperatures. Lead fouling appears as a different (often gray/tan glazed) deposit, and oil fouling produces a wet, oily coating.
Why the other options are wrong
- B. This deposit pattern indicates an abnormal condition, not normal operation.
- C. Lead fouling typically appears as a different colored, sometimes glazed deposit, not dry black soot.
- D. Oil fouling produces a wet, oily residue, not a dry fluffy black deposit.
Spark Plug Fouling Types
Deposits on spark plug electrodes that indicate specific engine operating problems.
- Carbon fouling: dry black soot from rich mixture or idling
- Lead fouling: gray/tan deposits from leaded fuel additives
- Oil fouling: wet oily coating from excessive oil consumption
Memory trick: 'Black and fluffy means too much fuel is stuffy'