Sparkplugs have an ideal temp at which they like to run, thus the various heat ranges of plugs. The heat range is governed by what the engine is designed to do so the plug temp easily starts the "burn". If the plug is to cold it is hard to start the burn and if it is to hot it can start the burn before the spark gets there
preignition. Also the "boots" on the plugs are part of the design to regulate the cooling rate of the plug and keeping it warm. Boots are important in most applications!!
Now a cold or cooler plug when the engine is running can be 2 things, either the cylinder has a malfunction, like fuel mix, sticky valves or vacuum leaks. If the cylinder is compromised it will not make the power which is the heat thus a cooler plug. The other is the ignition side. If there is a weak spark or a compromised wire that spark can be weak, it will not lite the burn very well and not create much heat (power) and thus a cooler feeling plug.
Dave Neptune
