A mixture that’s too lean will do it because the gasoline drawn into the firing chamber has a cooling effect. If there is not enough gasoline, there is not enough cooling. When a spark plug gets too hot, the insulator may boil and bubble On examination, it will be plain that it has been too hot. Also, the metal electrodes may melt away and disappear. If any of these bad things happen, that’s good.
The worst-case scenario of a too-hot plug that fails to destroy itself is when it destroys the engine instead; this is called pre-ignition. If the tip of the plug becomes hot enough to ignite the fresh mixture being drawn into the cylinder, then the incoming mixture will start to burn without waiting for the spark to happen.
Ignition due to any hot spot in the cylinder begins before the proper time for ignition, so it is called pre-ignition. Anything in the combustion chamber which gets hot enough can cause pre-ignition, but typically the end of the spark plug is the cause. When the mixture is firing sooner than it should, that’s like advancing the spark too much, and no matter what causes it, early ignition makes engines heat up, causing pre-ignition. Eventually, something melts, which comes under the heading of a bad thing.
What happens when a plug is too cold?
If the nose of the plug is not hot enough, it will gradually accumulate deposits, known as fouling. During normal engine operation, residue from the combustion process hits the insulator nose. This may include carbon, unburned fuel and oil, and chemical additives present in both fuel and oil. If the insulator nose and electrodes are hot enough, the combustion deposits will be continuously burned off by the heat of the plug. The ideal situation is to have the deposits burned off as fast as they accumulate, so the insulator nose stays fairly clean and free of deposits.
If the deposits accumulate on the plug because it is not reaching a high enough temperature to burn them off, the gradual accumulation will eventually short out, or foul, the plug. The fouling is electrically conductive and forms a path along the insulator, which connects the center electrode to the metal shell of the plug.