vapor lock

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A pocket of vaporized gasoline in the fuel line of an internal-combustion engine that obstructs the normal flow of fuel.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a stoppage in a pipeline caused by gas bubbles (especially a stoppage that develops in hot weather in an internal-combustion engine when fuel in the gas line boils and forms bubbles that block the flow of gasoline to the carburetor)