FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesMedium
A technician is troubleshooting a reciprocating engine that exhibits consistent afterfiring (misfiring and combustion in the exhaust system) during engine shutdown. Which of the following is the most likely cause?
- AExcessively lean idle mixture.
- BIncorrect magneto timing.
- CFouled spark plugs.
- DExcessively rich idle mixture.
Show answer & explanationAnswer & explanation
Correct answer: A. Excessively lean idle mixture.
Afterfiring, also known as 'dieseling' or 'run-on,' occurs when an engine continues to fire after the ignition is turned off. An excessively lean idle mixture can cause this by allowing hot spots in the combustion chamber or exhaust system to ignite the lean fuel-air mixture, leading to uncontrolled combustion.
Why the other options are wrong
- B. Incorrect magneto timing would primarily affect engine performance, starting, and possibly lead to backfiring or detonation, but is less likely to be the direct cause of afterfiring during shutdown.
- C. Fouled spark plugs would cause misfires and rough running, but afterfiring occurs due to uncontrolled combustion after ignition is off, not necessarily due to spark plug condition alone.
- D. An excessively rich idle mixture would typically lead to rough running, carbon fouling, and possibly black smoke, not afterfiring.
Afterfiring (Dieseling)
Combustion that continues in an engine after the ignition system has been turned off, often characterized by rough, uncontrolled firing.
- Occurs after ignition is OFF.
- Caused by hot spots igniting fuel.
- Often due to lean idle mixture.
Memory trick: Lean hot spots ignite, causing run-on, like a campfire after the match is out.