OVERHEATING
It sometimes happens that the engine will continue to run after the ignition circuit is opened. This may be due to a failure of the water circulation, which is indicated by the low temperature of the radiator, or by the low speed or stopping of the fan from a slipping or broken belt. If the cooling system is working properly, the lubrication must be investigated. If the oil is flowing as it should, the cause will be found in the formation of a carbon deposit in the combustion space, fine points of which will become incandescent and ignite the mixture as it passes into the cylinder or as it is compressed. A more unlikely fault may be that the points of the spark plug are fine enough to become heated in a similar manner; modern plugs are made with points of such size that this possibility need hardly be considered. This ignition of the mixture by other cause than the proper spark is termed preignition.