What can the use of excessively higher grade of fuel cause?

• A. Higher octane improves throttle response.
• B. Increased fuel consumption.
• C. Low temperatures and plug fouling.
• D. Increased power output.

Answer: (C)

Using a fuel with an octane rating much higher than what the engine needs changes how the mixture burns. Higher octane resists detonation, so if the engine isn’t benefiting from that property, combustion can be less complete, especially when the engine is cold or not fully warmed up. That incomplete combustion can leave carbon and unburned fuel on spark plugs, leading to plug fouling and cooler operating temperatures in the cylinder. The other effects, like better throttle response or more power, don’t reliably occur with simply using a higher-grade fuel, and octane rating doesn’t directly increase fuel consumption.