What action should be taken before restarting a runaway diesel engine?

Before restarting a runaway diesel engine, it is crucial to ensure that the air supply is cut off. A runaway condition refers to a situation where the engine is revving uncontrollably, often due to an excessive amount of fuel entering the combustion chamber or an air leak that keeps the engine running beyond its normal capacity. By cutting off the air supply, you effectively prevent the engine from continuing to run, giving you the opportunity to safely address the underlying issue.

In this context, checking the oil level, inspecting the valve system, or reattaching the fuel line would not directly address the immediate need to stop the engine from running uncontrollably. The focus must be on mitigating the runaway situation by eliminating the air supply, allowing for safe inspection and potential repairs once the engine is brought back under control.