What is the purpose of a fuel tank breather, and what problem can occur if it becomes blocked?

A. The breather has no functional purpose and exists only for regulatory compliance

B. The breather allows air to enter the tank as fuel is drawn out (and escape as fuel expands) — if it becomes blocked, a vacuum can form in the tank as fuel is consumed, which can restrict or stop fuel flow to the engine, sometimes only becoming apparent once the engine has been running for a while

C. A blocked breather only affects how quickly a tank can be refilled, nothing else

D. Breathers are fitted only to petrol tanks, never to diesel tanks

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

B. 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

C. A manufacturing defect specific to certain diesel engine brands

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 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

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

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