* Over-tightening: This is a very common cause. Over-tightening the spark plug can stress the ceramic insulator, leading to cracks, especially at the base of the insulator near the threads. The #4 cylinder location might not be easily accessible, leading to more forceful tightening or the use of improper tools.
* Heat cycling: Repeated exposure to high temperatures (during combustion) and then cooling can cause thermal stress cracks in the ceramic insulator. This is exacerbated if the engine is running lean (not enough fuel) or if the cooling system is not functioning properly, leading to higher operating temperatures.
* Pre-existing damage: The spark plug may have been damaged before installation, perhaps during shipping or handling. A microscopic crack might not be visible initially but could propagate over time.
* Poor quality spark plug: Using a lower-quality spark plug that's not designed for the engine's specifications can increase the risk of cracking. The insulator may be more brittle or thinner.
* Engine vibrations: Excessive engine vibrations can put stress on the spark plug, increasing the risk of cracking, especially in an older vehicle where engine mounts might be worn.
* Incorrect gap: An incorrectly gapped spark plug can cause excessive heat and stress on the insulator.
* Corrosion: Corrosion around the threads can contribute to a weakened spark plug and make it more susceptible to cracking.
* Carbon buildup: Excessive carbon buildup on the spark plug can contribute to higher operating temperatures and increase the risk of cracking.
To determine the exact cause, a mechanic would need to inspect the engine, the cracked spark plug, and potentially the surrounding components. Replacing the plug with a proper specification spark plug, properly torqued, is essential. Addressing any underlying issues like cooling system problems or engine mounts will prevent recurrence.
