Changing a water pump impeller

Changing a water pump impeller is normally a straightforward job that every voyager should be able to tackle.  Impeller pumps are far more common than you might imagine and are used for everything aboard from bilge pumps to shower pump and freshwater wash down. Generally they can be relied upon to work without incident for many months or even years but that does not mean they are maintenance free. In my view one of the most important of these pumps is that used for raw water cooling on the inboard engine. Should this pump fail or work at less than full capacity overheating will result and damage result to the engine. I am sure that I am not the only one who has a quick peek over the transom after starting the engine to make sure that water is being ejected with the exhaust. All engines differ but if you get used to a quick visual check each time the engine is fired up you will soon spot if something is amiss; reduced water flow or a dry throaty exhaust note is a sure sign that something is amiss. I must confess that I like to change my raw water impeller every winter no matter how little I may have used the boat during the season. Largely a preventive maintenance issue 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 left for too long before being serviced and are apt to fail at the most critical times just when you need the engine the most.

Here’s how to do it.

This is a typical set up which in this case is a Yanmar 3GM engine with the raw water pump driven by a belt from the from end of the crankshaft. All engines vary to some extent and some pumps are driven by gears or pinion shaft in which case they are bolted directly to the engine block.

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 and that is to shut off the water at the seacock especially if the water pump is lower than the water level as the siphon action can flood the boat. Undo the pipe clamps after removing the securing nuts and bolts pull the pump clear.

With the pump on the bench or saloon table undo the bolts or screws that hold the cover plate in place. These are often small so it is 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.

With all the bolts removed lift off the cover plate, if it is stuck in place insert the blade of a sharp knife into the joint to pop it free.

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

If you do not have a pair of pliers to hand or the impeller is well and truly stuck use a couple of flat bladed screwdrivers. Use them at 180 degrees to each other in the manner shown and lever out the impeller.

With the impeller out of the way now is a good time to inspect the pump housing. Give it a good wipe out with some clean rag and check for deep scoring or other damage. Small scratches and dings are unimportant but if the body is badly corroded or worn consider a replacement.

The paper gasket will probably remain stuck either to the pump body or the cover plate when it was removed. Use a razor blade or sharp knife to clean it off, don’t be tempted to reuse the old gasket 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 have cut a new gasket from an old chart.

With everything cleaned up we are now simply working in reverse order. Smear a little glycerin onto the inside of the pump body.

Then a little more onto the vanes of the impeller. The glycerin has two jobs, it makes the impeller slip into the housing and it also proves much needed lubrication for the 30 seconds or so until the cooling water starts to flow through the pump.

Slide the new impeller into place aligning the drive pin with the slot in the spindle, then 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. 
Some people advocate the use of a nylon cable tie to prebend the vanes to make insertion easier. I have never found this necessary but it is a good trick for larger pumps with heavy duty impellers.

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

Finally fit the pump back onto the engine and secure it with the nuts and bolts that you removed earlier, don’t forget to 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.

Additional information and bits of interest are included below for your amusement

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 stated it was not working quite as it should!

A selection of common raw water pumps. From left; a Jabsco spindle driven pump from a Perkins engine, a Volvo model and a Johnson gear driven pump. All can be serviced in the same way described in this feature.

Tip

After several seasons the inside of the cover plate may start to show signs of wear from the constant rubbing of the edge of the impeller, this in turn lets water seep by reducing the efficiency of the pump. Before rushing out to try and find a replacement try turning the plate over as most as symmetrical. Clean off any paint and polish with some fine sandpaper before refitting the wrong way round and you will find that you can extend the life of the pump by several years.

Comments

[amastron]

Another tip if I may, If the impeller is missing blades as in the picture. Look down stream an try to find ALL the missing blades. Heat exchanger is a good place to start!

Thanks....

[Michael Reardon]

Nice article Mark. I like the ziptie and cover flip tricks, may add those to the bag of tricks. I've always used silicone grease b/c it's handy- same stuff used for dive gear. Does the glycerine hold up well for the season? Also, those impeller pullers are pretty gentle if you wish to cling to that old impeller for a spare.

[mark]

The lubricant is only there to help in the initial installation and period when the pump runs at start up until the water comes through. After this time the vanes are lubricated by the water. The glycerine is water soluble and is washed out within a couple of seconds of pump running. No grease should be used that is going to stay on the vanes which may actually be damaged by grease.

[Pond Pumps]

Usually it is a good idea to clean the contact surfaces and remove any crud on the input side or it could break off and damage the turbine at a later date. A little silcone grease on the waterproof seal can improve things a little too..

I found this article very useful, thank you.

[Andy]

As ever, highly informative and very helpful! I have a Yanmar 1GM10 which is overheating. The inflow into impellor chamber seems good. I replaced the impellor which was worn but in one piece and this has not resolved the problem. It has been suggested that the cause is limescale build up which is preventing the flow of cooling raw sea water. Is this a common problem and how is it best rectified? Thanks!

[Mark]

Several things can cause an engine to overheat. The first would be to check and replace the impeller as you have done, the next would be to check the water flow through the engine. With the engine running there should be a good flow of water being pushed out of the exhaust as the engine runs, if not that would indicate either a blockage in the water pickup pipe or raw water strainer which must be clean and clear or a blockage inside the water galleries inside the actual engine, which is quite rare.
Check the obvious first my money would be on a restriction either in the raw water seacock or the strainer so check these initially. It is unlikely that the problem is in the actual engine itself. The last and final thing that is often overlooked is if the engine is only overheating as it is pushing the boat along there could be a lot of weed or even a bit of rope around the prop which is placing too much strain on the engine and gearbox.

[Andy]

Mark,thanks again for your advice. As it turned out it was indeed an obstruction caused by the accumulation of salt deposits that completely occluded part of the cooling system. A giveaway is a 'cracking' sound which can also be felt when the rubber hose components of the cooling system are squeezed. Clearly a pitfall of raw sea water and long periods of non use of the engine, although I have been careful to run it at least once a month for half an hour ...

[eva emilin]

Glad to see your article. One thing to note is that impeller pullers are pretty gentle if you wish to hold onto to that old impeller for a spare.

[Planche jean jacques]

excellent traitement du sujet ,bravo et merci

[Ross Elliott]

Another common cause of an engine " running warm" often overlooked is if the boat is OVERPROPPED either by diameter or pitch.Things usually are ok at low RPM but when you open it up a bit that's when this problem occured.Got the tee shirt on that one I'm afraid learned the hard way.