Vapor lock is a problem caused by liquid fuel changing state to gas while still in the fuel delivery system of gasoline-fueled internal combustion engines. This disrupts the operation of the fuel pump, causing loss of feed pressure to the carburetor or fuel injection system, resulting in transient loss of power or complete stalling. Restarting the engine from this state may be difficult.

The fuel can vaporize due to being heated by the engine, by the local climate or due to a lower boiling point at high altitude. In regions where fuels with lower viscosity (and lower boiling threshold) are used during the winter to improve engine startup, continued use of the specialized fuels during the summer can cause vapor lock to occur more readily.