text: what component in diesel fuel causes it to expand/ grow? diesel fuel expands when placed in the sun or heat.  grows about an 1/8 of a gallon.  what molecularly causes this?  its not a living organism.  there has to be a reason. most substances expand when heated.  water is the only exception because of it's hydrogen bonds (with oxygen).  diesel fuel is composed of mostly straight chain hydrocarbons with relatively low boiling points meaning they are not very attracted to each other and are likely to spread out when heated.