This condensed water, now in droplet form, runs down into the fuel due to gravity. The result is hard starting, miss firing, engine damage, and poor performance. Engine Knocking As low boiling molecules are lost though evaporation, a decrease occurs in the fuels Anti-Knock Index. This Index is the average of the R.O.N. and the M.O.N. octane numbers. As everybody knows, low octane means your engine will knock.
Engine knock is the result of combustion chamber pre-ignition.
In other words, your piston is still traveling upwards when the fuel mixture ignites. The result is severe engine damage. As the temperature of your fuel decreases, air from the surrounding environment is drawn into the fuel tank through the vent hose. This air contains thousands of unique, destructive particles. The result is damaged cylinder walls.