Which symptom would most likely indicate that the idle mixture is too rich?

• A. Spark plug fouling
• B. Idle RPM drops
• C. Lean misfire
• D. Overheating

Answer: (A)

When the idle mixture is too rich, there’s more fuel than the engine can burn at idle. The extra fuel often doesn’t combust fully and travels to the spark plugs, leaving carbon/sooty deposits on the electrodes. This spark plug fouling is a direct sign that the mixture contains too much fuel, because the deposits interfere with the spark and ignite poorly, especially at idle when combustion is most sensitive.

That’s why spark plug fouling is the best indicator. A lean misfire points to too little fuel, not too much; overheating is more associated with lean conditions or other faults; and idle RPM changes can occur for several reasons but aren’t as specific to a rich idle mixture.