Which of the following are causes of fuel vapor lock?

• A. Low fuel pressure, high fuel temperature, and excessive fuel turbulence.
• B. Low ambient air pressure.
• C. Too high octane rating.
• D. Water contamination.

Answer: (A)

Vapor lock happens when fuel in the delivery system turns into vapor and forms bubbles that block the flow to the engine. Keeping fuel in liquid form requires sufficient pressure and not too high a temperature. If the fuel gets hot, its tendency to vaporize increases; if the pressure in the line drops, it's easier for a bubble of vapor to form. When the flow is also very turbulent, those vapor pockets are more likely to persist and travel toward the pump or carburetor, causing a interruption in fuel delivery.

Therefore, the situation described—low fuel pressure, high fuel temperature, and excessive fuel turbulence—creates the conditions most likely to produce vapor lock.

Other options don’t fit as the primary cause: low ambient air pressure isn't the main factor in most fuel systems for vapor lock, high octane rating doesn't promote vaporization (it's about knock resistance), and water contamination leads to other fuel-system problems rather than direct vapor lock.