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Reading the Symptom: Won't Start, Overheating, No Drive
💡 Match the symptom to the system — won't-fire means fuel, overheating means raw-water cooling, no-drive means gearbox or prop — then check that system in order.
Recall the diesel's three needs from earlier cards: clean fuel, air, and compression — with raw water carrying heat away. Diagnosis is just working backwards from a symptom to whichever of these has failed. A calm skipper reasons; a panicked one starts replacing parts at random.
Won't start (turns over but won't fire) is almost always fuel, not electrics — a diesel has no spark plugs to fail. Suspect a clogged fuel filter, a near-empty or dirty tank, or air in the fuel line after running dry or changing a filter (the fix is to bleed the system). If it won't even turn over, now it's electrical: flat battery, loose terminal, or the engine stop control still pulled out.
Overheating points at the raw-water cooling circuit. Trace it in order: seacock open? strainer clear of weed or plastic? impeller in the raw-water pump intact? drive belt tight? A quick tell — no water spitting from the exhaust means no raw water is flowing, so stop the engine before you cook it.
No drive — engine revs happily but the boat doesn't move — isolates the problem aft of the engine: a gearbox fault (low oil, slipping, or a disconnected shift cable) or a fouled propeller, classically a mooring line wrapped around the shaft.