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Water Pump Impeller Service

If cooling water is the life blood of the engine, the impeller is its heart. A regular checkup of the water pump is just what the doctor ordered.

Selection of Raw-Water Pumps

A selection of common raw-water pumps. From left to right: Jabsco spindle-driven pump from a Perkins engine, a Volvo model, and a Johnson gear-driven pump. All of these can be serviced in the way described in this article.

Changing a water-pump impeller is normally a straightforward job that every boater should be able to tackle. Impeller pumps are far more common than you might imagine and are used for everything aboard from bilge and shower pumps to freshwater and seawater washdowns. Generally, they can be relied upon to work without incident for many months or even years, but that doesn't mean they're maintenance free.


If the inside of the cover plate shows wear, try reversing it, as most are symmetrical. Clean off any paint, and polish the plate with fine sandpaper before refitting. This can help extend the life of the pump by several years.

One of the most important of these pumps is for raw-water cooling on an inboard engine. Should this pump fail or work at less-than-full capacity, overheating may result, causing damage to the engine. It's a great idea to get into the habit of looking at the exhaust outlet after starting the engine to make sure water is flowing. All engines differ, but if you make a quick visual check each time the engine is fired up, you'll be more likely to quickly spot something amiss: Reduced water flow, excessive steam, or a dry, throaty exhaust note is a sure sign that something isn't working as it should.

I like to change my raw-water impeller every winter no matter how little I may have used the boat during the season. This is largely a preventive maintenance issue, and I then can relax knowing that when the boat goes back into the water, the water pump is in tiptop condition. Often water pumps are ignored for too long and are apt to fail at the most critical times.

  1. STEP 1

    This typical setup is a Yanmar 3GM engine with the raw-water pump driven by a belt from the end of the crankshaft. All engines vary to some extent, and some pumps are driven by gears or pinion shafts; these are bolted directly to the engine.

    Raw-Water Pump

    Depending on the engine/pump configuration, it can often be easier to remove the pump from the engine to change the impeller. One important caveat here is to shut off the water at the seacock. Undo the pipe clamps, and after removing the securing nuts and bolts, pull the pump clear.

  2. STEP 2

    With the pump on the bench or saloon table, undo the bolts or screws that hold the cover plate in place.

    Removing Bolts from Cover

    These are often small, so it's a good idea to store them temporarily in a cup or can, as they can easily roll into the bilge, never to be seen again.

  3. STEP 3

    With all the bolts removed, lift off the cover plate. If it's stuck in place, insert the blade of a sharp knife into the joint to pop it free or try gently tapping the edge of the plate with a small rubber mallet or the wooden end of a screwdriver.

    Removing Pump Cover Without Damaging O-Ring

    These tricks help you avoid cutting the O-ring seal that may be used instead of the paper gasket on some pumps.

  4. STEP 4

    I like to use a set of channel lock pliers for removing the old impeller.

    Removing Old Impeller

    Grasp the central portion of the impeller and pull it free. You might have to wiggle it a bit to release it from the spindle. Many DIYers prefer a dedicated impeller-pulling tool that fits the pump.

  5. STEP 5

    you don't have a pair of channel lock pliers at hand, or the impeller is well and truly stuck, use a couple of flat-bladed screwdrivers.

    Removing Old Impeller with Flat Screwdrivers

    Use them at 180 degrees to each other, in the manner shown, to lever out the impeller. Take care not to mar the edges of the pump.

  6. STEP 6

    With the impeller out of the way, now is a good time to inspect the pump housing.

    Inspecting Pump Housing

    Give it a good wipe out with a clean rag, then check for deep scoring or other damage. Small scratches and dings are unimportant, but if the body is badly corroded or worn, consider replacing the pump. Most pumps have a raised interior portion to flex the impeller blades. This can be replaced in some pumps; do this if it's worn or the edges are sharp.

  7. STEP 7

    The paper gasket will probably remain stuck either to the pump body or to the cover plate when it's removed.

    Removing Old Paper Gasket

    Use a razor blade or sharp knife to clean it off; don't be tempted to reuse the old gasket, as it will almost certainly leak, and unless the cover-plate seal is airtight, the pump won't work correctly. In the absence of anything else, I've cut a new gasket from an old chart. Instead of the paper gasket, some cover plates have an O-ring, which should be renewed.

  8. STEP 8

    With everything cleaned up, we're now simply working in reverse order.

    Smear Glycerin Inside Pump Body

    Smear a little glycerin onto the inside of the pump body.

  9. STEP 9

    Then smear a little more onto the vanes of the impeller.

    Smear Glycerin on Vanes of Impeller

    The glycerin has two jobs: It makes the impeller slip into the housing, and it also provides much-needed lubrication for the 10 seconds or so until cooling water starts to flow through the pump.

  10. STEP 10

    Slide the new impeller into place, carefully aligning the drive pin with the slot in the shaft and making sure the vanes face in the right direction. Some pumps instead have splines on the shaft that match corresponding splines inside the impeller. Set a new gasket in place making sure that the slight bulge on the inside coincides with the cam on the inside of the pump body.

    Inserting New Impeller and Replacing Paper Gasket

    Some people advocate the use of a nylon cable tie to prebend the vanes to make insertion easier. I've never found this necessary. It could be a good trick for larger pumps with heavy-duty impellers.

  11. STEP 11

    Refit the cover plate and snug the screws or bolts, being careful not to tear the gasket.

    Refit Pump Cover Plate

    Tightening the screws in a star pattern can help you avoid warping the cover, which may cause the pump to leak.

  12. STEP 12

    Finally, fit the pump back onto the engine and secure it with the hardware you removed earlier.

    Fit Pump Back Into Engine

    Then reconnect the hoses. If the pump is belt-driven, there should be about half an inch of deflection in the belt. Too slack, and the belt will slip; too tight, and the pump bearings will wear prematurely.

Guess which is the new impeller?

Pumps stand up to a lot of use and abuse; when this pump was dismantled, it was still working with just one blade left intact, although the owner admitted that it wasn't working quite as it should have been!

New and Old Damage Impellers

Anytime you have a missing or partially missing blades, check downstream of the pump for its remnants, as they may obstruct the cooling flow.

Tech Support

Degree Of Difficulty


  • Channel lock pliers
  • Screwdrivers
  • Wrenches


  • Clean rags
  • Glycerin
  • Paper gasket
  • Impeller


This project will take approximately 1-2 hours


Your only cost will likely be the price of a replacement impeller. Prices vary depending on the size and type of your pump.

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Mark Corke

Contributing Editor, BoatUS Magazine

A marine surveyor and holder of RYA Yachtmaster Ocean certification, BoatUS Magazine contributing editor Mark Corke is one of our DIY gurus, creating easy-to-follow how-to articles and videos. Mark has built five boats himself (both power and sail), has been an experienced editor at several top boating magazines (including former associate editor of BoatUS Magazine), worked for the BBC, written four DIY books, skippered two round-the-world yachts, and holds the Guinness World Record for the fastest there-and-back crossing of the English Channel — in a kayak! He and his wife have a Grand Banks 32.