Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairMedium
During a routine inspection, a technician observes that the spark plugs on a vehicle are fouled with black, sooty deposits. The engine has been exhibiting symptoms of poor fuel economy and a rich running condition. Which of the following is the MOST likely cause for the spark plug fouling in this scenario?
- AConstricted air intake system.
- BIncorrect spark plug heat range (too hot).
- CAdvanced ignition timing.
- DExcessive oil consumption.
Show answer & explanationAnswer & explanation
Correct answer: A. Constricted air intake system.
Black, sooty deposits on spark plugs, combined with poor fuel economy and a rich running condition, are classic signs of incomplete combustion due to an overly rich air-fuel mixture. A constricted air intake system (e.g., clogged air filter) reduces the amount of air entering the engine, directly leading to a rich mixture.
Why the other options are wrong
- B. A spark plug that is too hot would tend to burn off deposits, leading to electrode erosion, not sooty fouling.
- C. Advanced ignition timing would generally lead to higher combustion temperatures and potentially pre-ignition or detonation, not typically a rich condition and sooty plugs.
- D. Excessive oil consumption typically causes oily, wet deposits, not dry, black soot from a rich condition.
Sooty Spark Plug Fouling
Black, dry, powdery carbon deposits on spark plugs, typically indicating an overly rich air-fuel mixture or incomplete combustion.
- Caused by too much fuel or too little air.
- Leads to misfires and poor performance.
- Common causes: clogged air filter, faulty O2 sensor, leaking injector.
Memory trick: Sooty plugs mean too much fuel, not enough air.