Smoke colour is a clue, not a diagnosis. Observe whether it occurs cold, hot, at idle, during acceleration or under steady load.
| Appearance | Common categories |
|---|---|
| Black | Too much fuel for available air, overloading, fouled propeller, intake or exhaust restriction, injector fault |
| White vapour when cold | Unburned fuel, low cylinder temperature, slow cranking; some visible water vapour can be normal |
| Persistent dense white | Misfire, poor compression, injector issue, coolant or water entry depending on smell and behaviour |
| Blue | Lubricating oil burning through rings, guides, turbo seals or overfilled crankcase |
| Grey | Mixed causes including incomplete combustion, load or oil; investigate context |
Steam can be mistaken for white smoke. Steam often disperses quickly and may accompany reduced raw-water flow or overheating. Smell, temperature and coolant behaviour help separate it from unburned diesel.
A brief black puff when accelerating an older mechanical engine can differ from continuous smoke at cruise. Persistent black smoke means the engine is overloaded or cannot breathe correctly and should not be ignored.
Do not diagnose a rebuild from colour alone. Record rpm, load, temperatures, fluid use and whether each cylinder contributes. Check simple restrictions and propeller condition before internal tests.