What problem is primarily prevented by the correct running down procedure?

The correct answer addresses the prevention of spark plug fouling through the proper running down procedure. When an engine is allowed to run down correctly after operation, it ensures that any residual fuel and combustion products are effectively burned off. This reduces the likelihood of unburned fuel accumulating on the spark plugs, which can lead to fouling. Spark plug fouling occurs when deposits form on the plug, causing ignition problems, misfires, and decreased engine performance.

By adhering to the recommended procedure, which often includes allowing the engine to idle for a brief period before shutdown, the combustion process continues to work effectively until the engine is turned off. This helps to maintain spark plug cleanliness and optimal engine function.

The other options refer to different issues that may not be directly linked to this running down procedure. For example, oil cooler coring and high rates of piston ring wear are influenced by other operational and maintenance factors, while over high temperatures on the next start-up can be managed through proper warm-up procedures rather than the shutdown process. Each of these issues requires specific attention and does not directly relate to the consequences of improper or neglectful running down of the engine after operation.