Your engine is cooled by seawater, and a small rubber impeller does the pumping. When it wears, cracks or loses a vane, less water flows, the engine runs hot, and a good day on the water ends at the dock. It is one of the cheapest parts on the boat and one of the most common reasons an engine overheats.
What the impeller does
On an inboard, a raw water pump driven by the engine draws water in through a seacock and a strainer. A flexible impeller inside the pump pushes it through the heat exchanger, or straight through the block on a raw water cooled engine, and out with the exhaust. On an outboard the pump sits in the lower unit above the propeller, and some of the water leaves through the telltale, the small stream at the back of the engine. On many sterndrives the pump is in the drive itself, not on the engine, and there is no seacock to close; the drive's manual gives the procedure.
The impeller is rubber or neoprene, and rubber ages whether or not you run the engine. It takes a set, hardens and cracks. Running dry, even for a few seconds, can strip its vanes.
Signs your impeller is failing
- A weak, uneven or missing telltale stream on an outboard, or less water than usual with the exhaust on an inboard.
- An engine temperature that creeps up under load, or an overheat alarm.
- Steam in the exhaust.
- The engine was started without water: the seacock was closed, or the boat was out of the water with no supply.
Any of these is a reason to look before you run the engine hard again. If the engine is already running hot, start with what to check when your boat motor overheats.
How often to replace it
Your engine's manual comes first. It gives the interval for your model, in engine hours and in years, whichever comes first, and the right figure differs from one engine to the next. Boats used in salt water, in shallow or silty water, or left unused for long spells wear impellers faster than the hours alone suggest.
Many owners replace it every season whatever the hours say, because the part costs little and a failure costs a day. Keep a spare aboard with its gasket or O ring, and write down the date and engine hours each time you change it. If your boat is on Yacht Masters Hub, log each change on her record so the date and hours are always to hand: add your boat free.
Before you start
Work on a cold, stopped engine with the key out. On an inboard, close the raw water seacock before you open the pump. If the pump sits below the waterline and the seacock will not close, stop and call a mechanic.
You need the right impeller for your pump (the pump's maker and part number are usually on its body or in the engine manual), a new cover gasket or O ring, the lubricant the impeller maker recommends, an impeller puller or two flat screwdrivers, a light, rags and a cup for the screws.
Inboard diesel, step by step
- Close the raw water seacock.
- Remove the cover plate screws and lift the plate off with its gasket or O ring. Keep the screws in the cup; they are easy to lose in the bilge.
- Pull the old impeller out with the puller, or lever it out evenly with two screwdrivers without scoring the housing.
- Count the vanes. If any are missing, the pieces have gone downstream and must be found, usually at the inlet of the heat exchanger or oil cooler. Left there, they block the water and the engine overheats again.
- Look at the inside of the housing and the cover plate. Deep grooves on either mean they need replacing too.
- Lubricate the new impeller with the lubricant its maker recommends, never with petroleum grease, which attacks the rubber.
- Push it in with the vanes trailing, bent back against the direction the pump turns, and with its key or splines lined up as the old one sat.
- Fit the new gasket or O ring and the cover plate, and tighten the screws evenly.
- Open the seacock, start the engine and check for a good flow of water with the exhaust within a few seconds. Then check the pump for leaks. If no water comes, stop the engine and look for a closed seacock, a blocked strainer or trapped air.
Outboard impeller replacement, step by step
On most outboards the pump is in the lower unit, so the job starts by taking the gearcase off. The order below is the outline; your outboard's service manual gives the bolts, the torque figures and the shift position for your model, and it comes first.
- Disconnect the battery, remove the kill cord and pull the spark plug leads, so the engine cannot fire when the drive shaft turns. Tilt the engine up.
- Drain the gear oil if you will change it at the same time, which many owners do.
- Note the shift position the manual asks for, usually neutral, and mark where the trim tab sits so it goes back the same way.
- Remove the gearcase bolts and lower the gearcase away from the midsection, keeping the drive shaft and shift shaft straight.
- Remove the pump housing. Note how the old impeller's vanes sit and where its drive key is.
- Check the housing liner and the wear plate under the impeller. Scoring or a groove means they are replaced too, which is why many makers sell the pump as a kit.
- Fit the new wear plate and gasket, the drive key, and the new impeller turned in the direction of the drive shaft's rotation so the vanes trail, then the housing.
- Raise the gearcase back into place, lining up the drive shaft splines and the shift shaft, and refit the bolts to the torque in the manual. Check that all gears shift before you go further.
- Refill the gear oil from the bottom plug until it comes out of the top one.
- Run the engine in the water or on a flushing attachment, never dry, and check the telltale for a strong, steady stream.
If you have not dropped a gearcase before, this is a fair job to leave to a dealer or mechanic, and an easy one to pair with the yearly lower unit service.