Gasoline would naturally be a vapour than a liquid in our normal atmosphere
I am very limited with my schoolboy physics but I'm sure somebody will comment who isn't. Water will evaporate also, but neither do in a sealed container. A stationary engine cylinder can be considered sealed can it not? Petrol can return to liquid and become heavy due to condensation albeit at a lower temperature than water. When a cylinder is not firing you can identify it from touch because it will be cold or cooler. Engineers always then go to the bottom plug first which will likely be wet if that is the problem plug.
I'm very open to knowing more on the physics that takes place in the cylinder prior to ignition. If I'm completely wrong in my current understanding then I will stand corrected.