vapor lock
A problem mostly affecting gasoline-fueled internal combustion engines, where the liquid fuel changes state from liquid to gas while still in the fuel delivery system, reducing the pump pressure and causing transient power loss or complete stalling.
vapor lock: when fuel boils before it should burn
Vapor lock occurs when liquid gasoline vaporizes inside the fuel lines or carburetor before reaching the engine's combustion chamber. This happens because fuel system pressure drops below atmospheric pressure or temperatures exceed the fuel's boiling point, typically around 40 to 220 degrees Celsius depending on fuel composition and volatility rating. When fuel vaporizes prematurely, the fuel pump can no longer push liquid; instead it moves vapor, which occupies more volume and creates an air pocket. The fuel injectors or carburetor receive insufficient pressure and flow, starving the engine of fuel.
Vapor lock strikes hardest in hot climates, at high altitudes, or after prolonged idling in direct sunlight. Summer-blend gasoline, formulated to reduce evaporative emissions, has higher volatility and boils more easily than winter blends. Fuel lines routed too close to the engine block or exhaust manifold, inadequate fuel line insulation, or a weak fuel pump that cannot maintain adequate system pressure all increase risk. Symptoms include rough idle, hesitation under acceleration, sudden stalling, and difficulty restarting while the engine is hot.
Prevention and remedies
Modern vehicles largely eliminated vapor lock through fuel injection systems that maintain higher fuel rail pressures (typically 40 to 60 psi for port injection, 200 to 300 psi or more for direct injection), continuous fuel circulation that cools the fuel, and return lines that send excess fuel back to the tank. Older carbureted engines required shielding of fuel lines, insulated fuel line sleeves, heat-resistant hoses rated for the application, and sometimes mechanical or electric fuel pumps with sufficient capacity. Some vehicles included vapor return lines that bled gaseous fuel back to the tank during operation. Allowing an engine to cool before restarting and parking in shade during hot weather remain practical defenses on vulnerable vehicles.
The term reflects the physical state change: liquid fuel locks the fuel pump by converting to vapor, which cannot maintain hydraulic pressure. Vapor lock was most prevalent in the 1960s through 1980s, when carburetors and low-pressure mechanical fuel systems dominated and vehicle underhood temperatures rose steadily. Modern fuel injection and electronic engine controls have made the condition rare in current production vehicles, though it remains a hazard in classics, motorcycles, marine engines, and small equipment operating in extreme heat or altitude.