Short answer
Start the engine and watch the exhaust outlet within the first ten to fifteen seconds. You want a steady, substantial pulse of water with the exhaust gas, not a trickle. If flow is weak, work backwards through the circuit: seacock, strainer, hose, impeller, heat exchanger, then exhaust elbow.
You will need: Torch, bucket, gloves
Raw-water flow is the single most informative thing you can observe about a marine diesel, and it takes fifteen seconds. It belongs in every start-up, every time.
What normal looks like
Water should appear at the exhaust within the first ten to fifteen seconds of starting, pulsing with the exhaust gas. Learn your own boat's normal at a given rpm — volume differs enormously between installations, so a photo or a mental picture of yours is worth more than any general description.
If water has not appeared by about thirty seconds, stop the engine. Running dry destroys an impeller in under a minute and can damage the exhaust hose.
Working backwards through the circuit
When flow is weak or absent, check in this order, because it runs from most likely to least:
- Seacock — open, and fully open.
- Strainer — clear, correctly seated, lid sealing. A lid that leaks air gives weak flow with no visible blockage.
- Intake and hose — blocked intake, growth, or a suction hose collapsing under vacuum.
- Impeller — see how to tell if an impeller is failing.
- Heat exchanger — scaled or blocked tubes, or debris from a previous impeller failure.
- Exhaust elbow — carbon and corrosion restrict the mixing point over years.
The Australian angle
Warm-water fouling is faster than schedules written for cooler climates assume. In the tropics and through summer in the subtropics, strainers and intakes want checking as routine rather than annually — see service by region.