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Raw Water Impeller Has Failed
The impeller is a rubber-bladed rotor that pumps seawater through your engine. It is a wear item, it fails without warning, and when it goes the engine loses its cooling immediately.
Is it safe to keep going?
No. Without the impeller there is no seawater flow, and a marine diesel without cooling water damages itself in minutes. The exhaust hose can also melt. Shut down.
How you know it has failed
- No water at the exhaust, or a sharp drop in flow
- Temperature climbing, or the overheat alarm
- Blades found broken off when the pump cover comes off
- Blades hardened, cracked, or taking a permanent set from sitting
- On a boat that has just been launched or run on the hard: almost certainly this
No water coming out of the exhaust
Likely causes, ranked
| Cause | How common | What it looks like |
|---|---|---|
| Run dry — closed seacock, blocked strainer, run on the hard | Very common | Blades melted, glazed or shredded after seconds |
| Normal wear | Very common | Cracked, hardened blades; some broken off |
| Taking a set over a layup | Common | Blades bent to one side, pumps poorly on the first start |
| Age, even unused | Common | Rubber hardened and cracked with no hours on it |
| Debris — sand, silt, shell | Common in shallow water | Abraded blades, scored wear plate |
| Worn cam or wear plate | Occasional | New impellers keep failing early |
The part that matters most
Every blade must be accounted for.
When blades break off they travel downstream into the heat exchanger inlet, the oil cooler and the exhaust elbow. Fit a new impeller without finding them and the engine overheats again within hours — and the new impeller gets the blame.
Count the blades on the old one. If your impeller has six and only five come out, the sixth is in your cooling system, and the job is not finished until it is found. That usually means opening the heat exchanger end cap.
This is the single best question to ask a mechanic about this job.
Impeller replacement · Heat exchanger service
What you can check yourself
On many engines this is a job an owner can do, and it is the most worthwhile one to learn. Close the seacock first. Inspect the wear plate and cam for grooving while the cover is off — a scored housing eats new impellers. Replace the cover gasket or O-ring; do not reuse it.
When to call a mechanic
If blades are missing and you cannot access the heat exchanger, or if the engine overheated before you shut down, or if new impellers keep failing — that last one means the pump housing itself is worn.
What marine engine work costs in Australia
Common on these engines
Every raw-water-cooled marine diesel. It shows up soonest on engines that sit — yacht auxiliaries and seasonal boats, where the impeller takes a set rather than wearing out.
→ Yanmar: impeller failure · Beta Marine: impeller failure · Nanni: impeller failure
Frequently asked questions
How often should the impeller be replaced? On schedule rather than on failure. Interval depends on water conditions and how long the boat sits as much as on hours — ask a local operator what they use.
My impeller looks fine — do I still replace it? If it is at its interval or more than a few years old, yes. Rubber hardens with age even unused, and a set impeller pumps poorly without looking damaged.
What happens if I leave a broken blade in the system? It restricts flow downstream, usually at the heat exchanger inlet, and the engine overheats again quickly.
I ran the engine with the seacock closed — is the impeller gone? Almost certainly, even if only for seconds. Replace it rather than hoping.
Why did a brand new impeller fail? Usually a worn cam or wear plate in the pump housing, or debris still in the system from the last failure.
Practical summary
Use this fault guide safely
Stop now
Shut down for oil-pressure alarms, overheating, fuel spray, smoke in the engine space or loss of cooling-water flow.
Check safely
Start with visible leaks, strainers, fluid levels, alarms and loose connections—only when the machinery is safe and isolated.
Record evidence
Note rpm, temperature, smoke colour, alarm text, sea state and exactly when the symptom began.
Escalate
Give the evidence and engine model to a qualified marine mechanic; avoid repeated restart attempts.
Sources and verification
This page provides general Australian guidance. The current manual for the exact engine, gearbox or drive remains the authority for procedures, specifications and service intervals.
- 1Seacock
- 2Strainer
- 3Pump & impeller
- 4Heat exchanger
- 5Exhaust outlet