Which of the following can lead to spark plug fouling?

City driving can lead to spark plug fouling primarily due to the stop-and-go nature of urban traffic. When a vehicle is driven in the city, it often operates at lower engine temperatures and does not reach optimal operating conditions as frequently as when driving on highways. This can result in incomplete combustion of fuel, causing carbon deposits to accumulate on the spark plugs. Additionally, frequent idling and short trips do not allow the engine to burn off excess fuel and moisture, further contributing to fouling.

In comparison, highway driving typically involves higher speeds and longer periods of continuous operation, which promotes complete combustion and helps keep the spark plugs cleaner. Similarly, driving long distances tends to maintain optimal engine temperature and efficiency, reducing the risk of fouling. While driving on dusty roads may introduce particulates into the engine systems, it is less directly linked to the condition of spark plugs compared to the operational conditions seen in city driving.