What to Do If You Put petrol in a Diesel Car 😲 What happens if you put the wrong fuel in. 😲 [EXPLAINED] what happens if you put the wrong fuel in petrol is a flammable liquid and burns quickly diesel is thick and oily and is combustible burning slowly in a petrol engine putting diesel into a petrol car will cause your car to break down as the engine can't burn the diesel since the diesel is oily and thick it clogs up the spark plugs and fuel systems meaning your engine will sputter to a stand