Lower fuel grade can cause which engine condition?

• A. Vapor lock
• B. Pre-ignition
• C. Lean misfire
• D. Detonation

Answer: (D)

Fuel octane rating describes a fuel’s resistance to detonation. When you use a lower grade, that resistance drops, so the air–fuel mixture is more likely to ignite prematurely under high compression or high power settings. This premature or explosive burning is detonation, which creates pressure spikes that can harm the engine. Vapor lock relates to fuel vapor forming in the fuel system and blocking flow, pre-ignition is ignition before the spark due to hot spots, and a lean misfire stems from too little fuel. But the key outcome of using a lower octane fuel is detonation.