Inboard & Sterndrive Impeller Replacement
Inboard impeller replacement is a scheduled item, not a repair — the vanes take a set and shed rubber into the cooling system whether or not the engine has overheated yet.
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⭐ X.X from N reviews · Done at your slip or lift · Published rates
First: where is your impeller?
This is the question that decides what the job costs, and a lot of owners don't know the answer.
Engine-mounted raw-water pump. Belt- or gear-driven off the front of the engine. Straight inboards, V-drives and Bravo-type sterndrives. The pump is accessible in the engine bay, and the job is comparatively quick.
Drive-mounted pump. On Alpha-type sterndrives the raw-water pump lives in the upper section of the drive itself — which means the drive has to come off to reach it. Same part, considerably more labour.
Tell us your engine and drive when you book and we'll tell you which you have and what it costs. A photo of the drive is usually enough.
"I have closed cooling, so I don't have an impeller"
A common and expensive misunderstanding.
Closed cooling doesn't remove the raw-water pump — it moves what the raw water cools. Instead of running through the block, lake water runs through a heat exchanger to cool the engine's coolant, then goes out the exhaust.
There is still a raw-water pump. There is still a rubber impeller. It still fails on schedule.
What closed cooling does protect you from is corrosion and freeze damage inside the block. It doesn't protect you from a failed impeller — and on a closed-cooling engine, shed vanes go straight into the heat exchanger, which is one of the more tedious things to have to clean out.
Why it's replaced on schedule, not on condition
The vanes take a permanent set. An impeller that has sat in one position for months keeps that shape. Bent over, it still moves enough water at idle to look fine — and then fails to move enough under load, which is exactly when the engine is producing the most heat.
Rubber degrades regardless of use. Heat cycles, age and sitting dry all harden it. A low-hours boat is not a low-risk boat.
A partial failure is worse than a total one. A total failure overheats immediately and you shut down. A partial failure lets the engine run slightly hot for a whole season, quietly cooking exhaust manifolds, risers and gaskets.
Where the shed vanes go — and why we chase them
When an impeller fails it doesn't simply stop. It sheds, and those pieces of rubber travel downstream with the water flow.
Every vane has to be accounted for. We count what comes out against what should be there. If pieces are missing, they're somewhere in the system and they will block something.
Where they end up:
- The heat exchanger on closed-cooling engines — lodged in the tube stack, restricting flow, presenting later as an overheat with a cooling system that tests perfectly
- The oil cooler or transmission cooler
- The block and manifolds on raw-water-cooled engines — the worst case, because they're the hardest to retrieve from
A shop that fits a new impeller without accounting for the old one has handed you a future overheat with no obvious cause. This is the single most common way this job gets done badly.
Signs yours is due or gone
- Temperature creeping up over a season, or running hotter than it used to
- Overheat alarm
- Weak or no water at the exhaust outlet
- A whine or squeal from the pump
- Rubber fragments found in the strainer or heat exchanger
- Nobody knows when it was last done — which is itself the answer
The job
1. Access the pump — in the engine bay, or by removing the drive on drive-mounted setups.
2. Impeller out, vanes counted. Anything missing gets traced before we go further.
3. Housing, wear plate and cover inspected. A scored housing means the new impeller won't seal properly — same argument as any pump.
4. New impeller, gasket or O-ring and cover. Fitted with the correct rotation and lubricant.
5. Raw-water strainer cleaned, hoses and clamps checked.
6. Run and verify — flow at the exhaust, correct operating temperature, no leaks.
Done at your slip or lift. Drive-mounted pumps may want the boat on a lift or trailer — same reasoning as bellows work.
While we're in there
- Bellows and gimbal bearing — if the drive is already off for a drive-mounted pump, this is the moment
- Thermostat and housing
- Hoses, clamps and the strainer
- Anodes and, on diesels, pencil anodes in the heat exchanger
- Heat exchanger inspection on closed-cooling engines
How often
On your engine manufacturer's published interval, and on time as well as hours — an impeller in a boat that sits degrades on the calendar.
The practical rule: if nobody can tell you when it was last replaced, it's due. It's part of annual service.
Tell us your engine and drive and we'll confirm the interval.
What it costs
Quoted per boat, because the two pump locations are genuinely different jobs — an engine-mounted pump and a drive-mounted one share a part number and very little else. How we quote.
Cheaper alongside annual service or bellows work, where the access overlaps.
Frequently asked questions
How often should an inboard impeller be replaced?
On your manufacturer's interval, and on time as much as hours. If there's no record of the last one, replace it now.
I have closed cooling. Do I still have an impeller?
Yes. Closed cooling changes what the raw water cools, not whether raw water is pumped. There's still a rubber impeller and it still fails.
Where is the raw-water pump on my boat?
Either engine-mounted at the front of the engine, or inside the upper drive on Alpha-type sterndrives. Tell us the engine and drive and we'll confirm — it makes a substantial difference to the cost.
What happens if pieces of the impeller break off?
They travel downstream and block something — usually the heat exchanger, sometimes an oil cooler, worst case the block. Every vane should be accounted for during the job.
My engine runs slightly warm but doesn't alarm. Could it be the impeller?
Very possibly. A partially failed impeller causes exactly that, and running warm all season damages manifolds and gaskets quietly.
Can this be done with the boat in the water?
Engine-mounted pumps, yes. Drive-mounted pumps mean removing the drive, which we'd rather do on a lift or trailer.
Is this the same job as an outboard water pump?
Same principle, different work. Outboard water pump replacement is here.
Should I keep a spare impeller on board?
On a cruising boat, yes — along with the tools and gasket. It's a genuine get-home item, and we'll show you where yours is if you want to be able to do it yourself in a pinch.
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