Which condition is most likely to contribute to vapor lock?

• A. High fuel temperature
• B. Low altitude
• C. Cold engine
• D. High fuel efficiency

Answer: (A)

Vapor lock happens when fuel in the lines gets hot enough to turn into vapor, creating bubbles that interrupt the continuous liquid fuel flow. When fuel temperature is high, the fuel’s tendency to vaporize increases, especially in long or heat-exposed lines near hot engine components. The pump then has to move vapor as well as liquid, which it can’t do effectively, causing a drop in fuel pressure and the engine to stumble or stall. That’s why high fuel temperature is the condition most likely to contribute to vapor lock.

Low altitude isn’t a direct trigger because higher ambient pressure doesn’t promote vapor formation in the same way heat does. A cold engine reduces the heating of fuel, making vapor lock less likely. High fuel efficiency isn’t related to the vaporization of fuel in lines.