What is the consequence of excessive scale in the cooling system?

The presence of excessive scale in the cooling system can significantly impact engine performance, particularly by increasing the risk of spark knock. Scale consists of mineral deposits that can accumulate in the cooling system, leading to reduced coolant flow and inefficient heat transfer. When these deposits restrict the cooling, the engine can overheat, causing abnormal combustion conditions known as spark knock or pre-ignition.

Spark knock occurs when the air-fuel mixture ignites too early during the compression stroke, which can result in engine damage over time. An engine that runs hotter due to inadequate cooling will be more prone to these knocking conditions, as higher temperatures can cause the fuel to ignite prematurely. Therefore, maintaining a clean cooling system free of excessive scale is crucial for sustaining optimal engine temperature and preventing spark knock, which is reflected in the correct choice.