Most inboard and outboard motors today are water-cooled. The impeller is one of the most important components. It is located in the water pump housing. The impeller circulates the water around to supply the engine with cooling water. It is therefore an important component that should certainly not be overlooked during maintenance. 

Although an impeller should last several years on paper, in practice we often see that it needs to be replaced sooner. This is often due to wear. A broken impeller always fails at an inconvenient time, so when you order a new one, always order a spare as well, including a gasket. 

Fortunately, replacing an impeller is quite simple and easy to do yourself. In this online guide, we will briefly show you how to do it yourself and what to pay attention to during installation. 

Which impeller do I need? 

Before you can begin installation, you naturally need a new impeller first. You will find many options online, but which one is suitable for your engine? To find out, it is best to check the brand, model, and serial number of your outboard or inboard motor. 

Below, we briefly cover the best methods: 

• Searching by engine detailsYou can often determine the correct impeller for your engine using the engine details by searching with the brand, serial number, and/or model number.

• Looking up the part number (OEM number): If you look up the manual or an exploded view of your engine, you will find the original part number (OEM number). With this number, you can order an original impeller directly or order a universal/replacement brand. 

• Measuring the old impeller: Have you already removed the old impeller and have no other information? Then you can simply measure the old impeller. Pay attention to the outer diameter, inner diameter, height/width, and type of shaft connection.

Replacing an impeller on an outboard motor

We begin with a step-by-step guide for an outboard motor. The rule of thumb for replacement here is: once a year or every 100 operating hours. Replacement usually takes 1 to 2 hours. 

What do you need? 

• New impeller (or a complete water pump rebuild kit including gaskets and stainless steel plate)

• Tools: Socket set, open-end wrenches, screwdrivers, and water pump pliers

• Lubricants: Water-resistant grease (marine grease or petroleum jelly)

• Optional: A container for storing bolts and a new cotter pin for the shift rod (if applicable)

Step 1: Making the engine safe

Remove the spark plug wires from the spark plugs. If the engine has an electric starter, also disconnect the battery. This prevents the engine from starting if you later turn the propeller or flywheel.

Step 2: Disconnecting the shift rod

On many engines, you must disconnect the shift rod before removing the lower unit. This rod controls forward, neutral, and reverse.

• Put the engine in neutral or forward gear, depending on the brand. Remember its position. It must be in the same position when reinstalled.

• Find the shift rod coupling. It is often halfway up the shaft behind a rubber cap or just above the lower unit.

• Loosen the coupling nut or remove the retaining pin to separate the rod.

Step 3: Removing the lower unit

The lower unit is secured with several bolts to the underside of the shaft.

• Mark the position of the trim tab (the zinc anode just above the propeller) with a marker. There is often a hidden bolt beneath this trim tab. Therefore, remove the anode first.

• Loosen the bolts in a crisscross pattern.

• Support the lower unit properly while loosening the last bolts.

• Carefully pull the lower unit straight down to remove it. The drive shaft comes out with it. On most engines, the cooling water tube remains in the shaft and slides out of the rubber grommet in the pump housing.

Step 4: Opening the water pump housing

The water pump is located on top of the removed lower unit, where the drive shaft enters.

• Loosen the 4 bolts securing the plastic or stainless steel water pump housing.

• Slide the pump housing upward over the drive shaft. Be careful not to damage the stainless steel inner sleeve in the pump housing.

• You can now see the old impeller.

Step 5: Removing the old impeller

• Slide the old impeller up and off the shaft.

• Pay attention to the direction of rotation. Carefully observe how the vanes of the old impeller are bent. The new one must be installed in the same direction.

• Watch out for the key (woodruff key). This is the small crescent-shaped metal piece in the shaft that makes the impeller rotate with it. Remove the key from the shaft without losing it, and clean the shaft thoroughly.

Step 6: Cleaning and inspection

• Remove the old gaskets and the stainless steel wear plate beneath the impeller.

• Clean the mating surfaces of grease and scale using a scraper or abrasive pad.

• Inspect the inside of the pump housing. Are there deep grooves? If so, replace the complete pump housing. Otherwise, the new impeller will not build up enough pressure.

Step 7: Installing the new impeller

• Place the new gaskets and the stainless steel wear plate in position.

• Apply a small amount of marine grease to the drive shaft and keyway. Reinstall the key.

• Lightly coat the new impeller and the inside of the pump housing with glycerin or silicone grease. This prevents the impeller from running dry during the first start. Do not use petroleum jelly or any other petroleum-based product, as it damages the rubber.

• Slide the new impeller over the shaft and align the slot precisely with the key.

• Slide the pump housing down over the shaft. While pressing it down, turn the drive shaft clockwise, viewed from above. The vanes will automatically bend in the correct direction. This also applies to engines with a propeller that rotates in the opposite direction, because the reversal is in the lower unit.

• Tighten the pump housing bolts in a crisscross pattern. Do not overtighten them, as the pump housing is often made of plastic. Follow the tightening torque specified in the workshop manual.

Step 8: Reinstall the lower unit

• Grease the splines (the teeth) at the top of the drive shaft. Never apply grease to the very top of the shaft. The shaft could then exert axial pressure on the crankshaft.

• Align the rubber grommet in the pump housing with the cooling water tube in the shaft.

• Carefully slide the lower unit straight upward. If the drive shaft splines do not immediately engage with the engine, turn the flywheel clockwise by hand. If the engine is in forward gear, you can also turn the propeller slightly. In neutral, the drive shaft does not turn with the propeller.

• Tighten all the lower unit bolts securely. Refit the trim tab in the marked position.

• Reconnect the shift rod exactly as it was. Test whether the engine shifts smoothly into forward and reverse.

• Reconnect the spark plug wires and reconnect the battery.

Step 9: Testing

• Never start the engine without cooling water.

• Use flushing muffs with a garden hose or place the lower unit in a large container of water. The water level must be well above the water pump housing.

• Turn on the water before starting the engine.

• Start the engine and check within a few seconds that a strong telltale stream is coming from the engine. Let the engine idle and do not run it at high speed on flushing muffs.

Replacing an impeller on an inboard engine

Replacing it on an inboard engine, such as a Volvo Penta or Yanmar, is often much easier and usually takes 25 to 30 minutes. The cooling water pump is mounted directly on or to the engine block, so you do not need to remove anything from the drive.

What do you need? 

• New impeller (including a new gasket or O-ring)

• Tools: Small flathead/combination wrench or screwdriver (for the cover), an impeller puller or water pump pliers

• Lubricant: The supplied glycerin/petroleum jelly (often included)

Step 1: Turn off the engine and close the seacock 

Turn off the engine and remove the key from the ignition. Then close the seacock for the cooling water supply. If you do not, seawater will flow into the boat as soon as you open the pump.

Step 2: Open the pump housing cover

• Locate the cooling water pump on the engine block. You can recognize it by the thick rubber hoses connected to it.

• Loosen the screws on the round cover. This is usually made of brass or stainless steel.

• Some residual water may run out of the pump. Place a cloth or container underneath.

Step 3: Remove and inspect the old impeller

• First check whether the impeller is secured to the end of the shaft with a screw or retaining ring. Remove it before pulling.

• Pull the old impeller straight out of the pump housing. Preferably use an impeller puller or water pump pliers. Be careful not to damage the edges of the pump housing.

• Pay attention to the direction of rotation. Check carefully which way the blades are bent.

• Check for missing pieces. Are rubber pieces from the old impeller missing? Look for them in the hose after the pump and at the heat exchanger inlet. Loose pieces of rubber can later cause blockages and overheating.

• Check the inside of the pump housing and the cam for grooves or wear. Replace the pump housing or cam if there is significant wear.

• Remove the old gasket residue from the cover and the pump rim.

Step 4: Install the new impeller

• Coat the new impeller and the inside of the pump housing thoroughly with glycerin or silicone grease. Do not use petroleum jelly or any other petroleum-based product, as it damages the rubber.

• Push the new impeller into the pump housing. Turn the impeller in the correct direction while pushing so that the blades bend the right way immediately.

• Make sure the impeller fits properly over the key, flat side, or splines on the shaft. This varies by pump type.

• Was the impeller secured with a screw or retaining ring? Reinstall it.

Step 5: Close the cover with a new gasket

• Install the new gasket or O-ring. Never reuse the old one; this prevents leaks.

• Do not use gasket sealant or a thicker gasket. The thickness determines the impeller clearance in the pump housing.

• Tighten the cover screws crosswise without overtightening.

Step 6: Open the seacock and test

• Open the seacock again. If you forget this, the new impeller will fail within a minute.

• Start the engine and immediately check at the exhaust outlet whether cooling water is coming out.

• Check the pump cover in the engine compartment for leaks.

Do you have questions or are you looking for a new impeller?

Do you have questions about installing your new impeller, or are you not quite sure which impeller you need? No problem! We are happy to help you with your search! You can always contact us if you are unsure about anything. See all details on our contact page