Your engine has started overheating. What is one of the most common causes worth checking first, and why?

A. The engine oil colour, since oil colour alone reliably indicates overheating causes

B. The battery voltage, since batteries directly control engine temperature

C. The colour of the exhaust smoke, which is unrelated to cooling in any way

D. The raw water intake and impeller — a blocked intake strainer, an airlock, or a failed/worn impeller all stop or reduce the flow of cooling seawater through the engine, and this is one of the most frequent overheating causes on small-craft diesels

A. A type of insect that occasionally gets into fuel filler caps

B. A manufacturing defect specific to certain diesel engine brands

C. A microbial contamination (bacteria and fungi) that grows at the interface between diesel and any water sitting at the bottom of the tank — it thrives especially in tanks that sit mostly full and undisturbed for long periods, and the resulting sludge can clog fuel filters and starve the engine, often at the worst possible moment

D. A problem that only affects petrol engines, never diesel

A. The engine oil colour, since oil colour alone reliably indicates overheating causes

B. The battery voltage, since batteries directly control engine temperature

C. The colour of the exhaust smoke, which is unrelated to cooling in any way

D. The raw water intake and impeller — a blocked intake strainer, an airlock, or a failed/worn impeller all stop or reduce the flow of cooling seawater through the engine, and this is one of the most frequent overheating causes on small-craft diesels