What Are the Symptoms of Vapor Lock in an Engine?

Vapor lock often causes a vehicle to fail to start after the driver shuts it off for a few minutes. It also causes cars to stall in slow traffic, when the engine tends to operate at a higher than normal temperature. The condition is caused by the heat in the engine compartment vaporizing the fuel in the fuel pump and lines.

Vapor lock occurs more commonly in carbureted vehicles and in air-cooled engines than in fuel-injection or water-cooled engines. Since the introduction of fuel injectors, most manufacturers have moved the fuel pump from the engine compartment to the fuel tank. Submersing the pump in the tank provides positive pressure in the fuel system and reduces the chances of vapor lock.

Operating a vehicle at a high altitude increases the chances of experiencing vapor lock. At the higher elevation, atmospheric pressure is lower, and the boiling point of gasoline is lower. Ethanol also has a lower boiling point than regular unleaded gasoline, increasing the likelihood of vapor lock.

Vapor lock affects the engines used on lawnmowers and aircraft. A vapor-locked engine in an aircraft can cause a forced landing. To prevent such emergencies in the air, most aircraft engines use fuel with a higher boiling point.