What does the use of fuel boost pumps help prevent?

• A. Vapor lock by keeping lines pressurized
• B. Vapor lock by increasing fuel temperature
• C. Vapor lock by reducing fuel pressure
• D. Vapor lock by atmospheric air intrusion

Answer: (B)

Vapor lock happens when fuel in the lines vaporizes due to heat and pressure loss, creating pockets of gas that interrupt the flow to the engine. The function of a fuel boost pump is to push fuel with enough pressure to keep those lines pressurized, so the liquid fuel continues to move and the engine can fed reliably, even when lines are hot or there’s extra demand. This keeps the fuel from flashing into vapor, which is the key to preventing vapor lock.

Thinking about the other idea—raising fuel temperature—would actually tend to increase vapor pressure and make vapor lock more likely, not less. Reducing pressure or allowing atmospheric air to intrude would also promote conditions that cause vapor lock rather than prevent them.