What is a common cause of a hole in a distributor cap?

A hole in a distributor cap is often caused by excessive secondary resistance. The distributor cap is part of the ignition system, and its main function is to route high-voltage current from the ignition coil to the correct cylinder's spark plug. When there is excessive resistance in the secondary circuit, it means that the energy that should be reaching the spark plugs is being impeded. This resistance can generate excessive heat in the ignition components and lead to arcing.

As a result, this arcing can create enough heat to burn or otherwise damage the plastic of the distributor cap, leading to the formation of holes. These holes can allow moisture and contaminants to enter the ignition system, further compounding issues related to spark delivery and engine performance.

In contrast, excessive primary resistance typically affects the coil's ability to create a strong magnetic field, but it doesn't directly result in physical damage like a hole in the cap. Similarly, conditions related to rich or lean air-fuel mixtures primarily affect combustion-related issues rather than the integrity of the distributor cap itself. Thus, excessive secondary resistance is identified as the probable cause for the hole formation due to the heat and electrical arcing it produces.