How to tell if your impeller has failed

Reduced or absent water from the exhaust is the symptom, and it is usually the first and only warning you get. A failing impeller does not announce itself in advance — it moves less water for a while, then sheds vanes and stops.

Check the exhaust flow every time you start. It takes two seconds and it is the most useful habit in marine engine ownership.

The signs, in the order you notice them

1. Reduced flow from the exhaust. Less water than you are used to seeing. This requires knowing what normal looks like for your boat, which is why the habit matters.
2. Temperature creeping up under load while idling normally.
3. A change in exhaust note — a wet exhaust sounds different with less water in it.
4. Steam or a burning smell from the exhaust, which means the raw water is no longer cooling it. Shut down.
5. No water at all. Shut down immediately — the exhaust hose and the mixing elbow can be damaged within minutes without cooling water.

Confirming it

Remove the pump cover and look — see how to change an impeller. What you find tells you what happened:

• Vanes missing — it has shed, and the fragments are downstream. Every one must be accounted for.
• Vanes flattened or taking a set — it has been sitting compressed, common after a layup.
• Vanes cracked at the root — age and heat.
• Glazed and hardened rubber — it has been run dry at some point.
• Intact but worn short — it no longer seals against the housing and moves less water than it should.

What it is not

Before blaming the impeller, rule out the cheaper and commoner causes of the same symptom:

• A closed or partly closed seacock
• A blocked strainer — see how to clean a raw water strainer
• A blocked or fouled intake outside the hull
• An air leak at the strainer lid, which lets the pump draw air instead of water
• A slipping or broken belt where the pump is belt-driven

If the impeller is intact and the flow is poor, the answer is upstream.

Why the new one failed too

An impeller that fails again within a season means something else is wrong:

• A scored wear plate or worn cam inside the pump
• Running dry, from any of the blockages above
• A weeping pump shaft seal

Replacing the impeller alone does not fix any of those.

The thing people get wrong

Not counting the vanes. A shed impeller sends its pieces into the heat exchanger tubes and sometimes the exhaust elbow. Fitting a new impeller without retrieving them leaves the debris in the circuit to cause the next overheat — and the owner blames the pump again.

Frequently asked questions

What is the first sign of impeller failure? Reduced water from the exhaust. Knowing what normal looks like for your boat is what makes it noticeable.

Can I keep running with a failing impeller? No. Without cooling water the exhaust hose and mixing elbow can be damaged within minutes, and the engine will overheat.

How long does an impeller last? Manufacturers commonly specify annually or at a set hour figure. It also takes a set if left compressed over a layup, regardless of hours.

Why did my new impeller fail so quickly? Usually a worn wear plate or cam in the pump, or running dry from a blocked intake or strainer.

What if some vanes are missing? They are downstream in the cooling system and must be retrieved, usually from the heat exchanger. Leaving them causes the next overheat.