Water Pump Impeller Change on Outboard Motors - How Often ?





Many Ideas Here ;  If you frequent about any of the outboard motor forums, you will often see newcomers asking for advice on when do they replace their water pump impeller?   Sometimes this can get to where the newcomer may feel like they are singled out and persecuted because of a simple question like this, if their impeller has not been replaced as often as the responder is blasting to the whole world to see. 


You will see many different ideas on water pump impeller replacement.  Some comments frequently being that you are insane or plain lucky if you have not replaced the impeller every year.    You will also see a recommendation to not just replace the impeller because everything wears, so to replace the compete water pump assembly.  Sometimes, the mindset seems to be "My mind is made up, don't confuse me with facts".


You have to remember that these are free message boards/forums and free advice is usually worth pretty close to what you paid for it.  Also some of these people who answer, may not be that knowledgeable when it comes to actual repairs, or more specifically the motor being discussed.  Sure they may have boated for a number of years and may have a high number of posts/answers on the forum, but that may not be significant when it comes to actual repairs because they take their's to a mechanic.  Some of these persons who respond may well be knowledgeable, however others may be just passing along hearsay, trying to up their posting numbers on the forum (ranking in their eyes).


On the photo below, the LH is a new water pump impeller.  Also seen here is the drive slot in the bottom for the impeller key which connects it into the driveshaft.  Some impellers like these do not use a woodruff or square key, but a round pin.  This round key fits in the recess of the impeller and then ON a flat of the driveshaft.  Flat you say, well it may because of a possible easier assembly.  The round key locked in 1/2 of the impeller, upon rotation of the driveshaft, binds the two together.


Here is a new impeller Both impellers are used, but of a different vintage showing the different hubs.  The one on the right has obviously taken a set & needed replacement  


I will be going out on a limb here, and am sure there will be critics to my ideas, but bear with me as I will try to use logic.   Personally for the average boater, every year seems overkill for replacing water pumps or impellers.   I have seen many impellers that have gone 10 years or more and still functioning fine.   One can assume that every person that owns a boat does not use it constantly even during the summer months.  So one boater may put 200 hours a year, while another may only make it out for a couple of days, and then the motor would not then be run constantly from daylight till dark either.  Therefore it would be hard to set a hard and fast amount of time between changes.


Impeller change is just like buying insurance, however do you really need a new one, maybe taking a closer look depending on a number of things, than just doing it like changing your smoke detector batteries every 6 months.


Motor manufacturers and dealers want to insure that you, the boater doesn't come back on them for any liability burned up motor, so yes, shorter replacement times will be recommended at least by them.   And if a dealer does yearly maintenance for you, I will bet that a impeller replacement will be included, for that same reason.  Plus they have no way of really knowing what environment the motor was ran in.   Just the labor to tear it down and inspect would be the same to reinstall the used one, for a savings of maybe $20 for a new impeller.  This would be different if you were doing the work yourself, but is the average boater capable or even wants to undertake that project.  If they did, would they even know what to look for as to remaining life of the impeller?


Then some boaters have no idea of even how to operate a boat, much less how the motor operates, as long has it has gas in the tank, a charged battery and someone did not forget to put the drain plug back in, they hit the lake for a ski party and just run it wide open.   Not saying the motor will not stand up under this type of use, but those type of boaters seem oblivious to many things around them, motor and trailer maintenance included.   They may be a very good accountant or office manager, or politician but have no clue on how mechanical things function.


You probably will see more impeller failures due to the owners running the motor with no water into the pump than failure due to situations encountered while operating the motor on a boat.  This could be just total negligence, not enough water in a barrel when testing, debris infested barrel testing water or simply forgetting to turn the garden hose valve ON when starting to use muffs.  Hell, I have even had a brain fart one day and forgot to turn the water faucet on, doesn't take long, 30 seconds will do it.


One day when we were pulling my boat out of the lake after a test run of changing props, across the slot from us another boater had unloaded and pushed their boat back to the middle of the dock.  The driver of the vehicle took off to the parking lot.  As the driver walked back down the dock to get into the boat, a "SKIPPER" hit the ignition and started the motor.  Boy did that motor sound loud.  I looked over, NO WONDER, he still had the lower unit/prop raised as during transporting, with the prop out of the water.  The vehicle driver got in and he shifted it into reverse but nothing happened, prop just fanning the air.  Looking back, he noticed the situation, then lowered the spinning prop into the water and reverse motion was attained.  This took almost 2 minutes and they acted like this was normal.  My partner, looked up from securing my stern to the dock and said "he just burned his impeller up".   They backed out away from the dock and took off up the lake, we pulled my boat out, but I always wondered if it indeed was ruined and they got stranded.  But maybe Mercury builds better impellers than the Johnson/Yamahas that I am used to working on.


Another time, the wife and I were looking at a used 70hp Johnson motor that was for sale.  When we got there the motor was still attached to the boat with it all sitting in his driveway.  Everything looked fine and I asked how does it run?  This guy reaches over and hits the starter, it starts immediately.  He had no water to the motor in any way.  I hollered to him to shut it down, his response was "That's OK, I ran it like this yesterday".  I immediately deducted $100 from my offer.  And yes, that impeller was totally chewed up.  This young man (30ish) had ran this boat for 10 years but apparently had no conception that it needed water to the motor if not in the water at a lake.


If you do your boating in muddy, sandy or debris infected water which could plug the water intake holes to the water pump, OR run aground occasionally, that would place your motor way higher on the "TO DO" list than a boater who only goes to a clear lake.


Also if you boat in an area where water can get dangerous at times, like crossing from a ocean to into a river bar where a failed motor (for any reason) could be life threatening, by all means change it yearly.


Yes, if it was my only power source and I was using it offshore 20+ miles, then that would be good insurance, but for the average boater on a lake, not the same situation at all.


On my motors, I have installed a small hour meter that just operates by a wire being wrapped around a spark plug wire.   This is a lot more accurate to how much use they get than just guesstimating.


This yearly replacement may have been viable when these types of impellers were invented 60 some years ago when all of this yearly replacement was probably conceived.  Another thing that seems logical, with modern technology and manufacturing , materials used on currently made impellers should be way better than those made 60 or to even 30 years ago,


Also, you will find that once a idea or mindset is published, right or wrong, it will be hard for that to be changed.  And those who continue to tote this are just passing on what they heard or read, not taking into account of a whole lot of encompassing conditions.

What about the new Evinrude E-Tec where they say NO maintenance for 3 years or 300 hours?  Spark plugs, water pump impellers, thermostats and gear oil are what they recommend changing at that time-frame.  I run a E-Tec and inspect/replace the impeller depending the number of hours each year, which is between 50 and 60.  So every 3 years seems about right for me.  Now to quantify this, I have a kicker motor that gets considerably more run time and I do run offshore at times so I understand the consequences.


Water Pump Operation :   The operation of most marine water pumps using a rubber vaned impeller is, the cavity between the vanes is what makes it pump water.  As the off-center impeller rotates, the cavity enlarges, drawing in a gulp of water.  As it continues to rotate, the cavity becomes smaller, and squeezes the water out and up the water tube to the powerhead.


These motors are equipped with a plastic/Nylon bodied water pump housing.  There is also a stainless bottom plate, this plate has to be positioned with the suction hole in the proper location/relationship as indicated in the photos.   If you are just replacing the impeller, you can tell which way is up by observing the wear of the impeller on this plate.   The impeller fits in this stainless cup liner which gives a very long lasting situation.   In most cases you will never need to replace the water pump assembly, just the impeller.   Sometimes the rubber impeller becomes un-bonded to the metal hub making for an inoperative pump.

Inspection : 
Check if there is any wear or deformities on the impeller, if so then replace it.  Also check for deterioration/scoring of the stainless steel base plate along and the housing cup that the impeller sits in for wear or grooves, you may consider replacing both the plate AND cup, OR the complete water pump instead of just the impeller.  Wear shown in the photo below could cause the pump to fail to pump water because of possible loss of sealing/suction.   The photo below is from a Yamaha T8.

Worn water pump cup & lower wear plate

Shown below on the left is the water pump with the impeller on the driveshaft.  During functioning the water is drawn into the housing in a slot in the lower plate of the water pump, (not shown here) but this slot is in the lower plate at the the center of the RH photo.   The vanes are not as compressed allowing the water to be sucked into the cavity.   As the impeller turns with the driveshaft, the vanes compress as the cavity is not centered , but offset or eccentric as shown in the LH photo.  This offset forces the water up into the rotating impeller and out the gap seen on the top of the cavity and up into the copper tube which is located in the 1/2" round hole in the top of the pump housing.  Here the tube is secured and sealed by a rubber grommet in the top of the water pump.


When the motor is running, the water pump is submerged, the water in the water pump housing is pushed by the rubber vaned centrifugal impeller upward and into the 3/8" copper supply tube up into the powerhead. 


This is a centrifugal pump, this means that the impeller is a rubber vaned insert setting inside of a stainless steel housing.  The impeller is offset to one side so that when the water comes into the pump housing at the widest part around the impeller and as the impeller turns the water is compressed and then forced out on the other side.   This is illustrated by the LH photo below.

The outer edges of the vanes are not the only significant part of the water pump impeller as the circular rubber rings on both sides also act a seal, keeping the water trapped between the vanes.

Water pump viewed from bottom side New water pump assembly



Below is research done using Mercury motors, others should be nearly the same as the water pump impellers are very similar for all outboards.


Damage can be done in just a very short time with no water to the water pump
of outboard motors


For the Do-It-Yourselfer :   Actual time to replace an impeller is not that much on most motors once you understand each motors technique.  An average replacement should be near or less than one hour.   For a link to actual replacement for a OMC  CLICK HERE, or for a Yamaha T8  CLICK HERE.   Others may be different, but you should get the idea after looking at these.


Things to look for after the water pump is removed are, the impeller can take on a curl rather than have the blades snap straight outwards when removed from the housing.  It may have also picked up some sand or some other type abrasive material which has created a scoring in the lower impeller plate and/or upper housing.  That scoring alone would have a tendency to create a air pocket creating inefficiency in the pump.  Any of these could be a contributing factor in cooling / not cooling of your motor.

Also on a recent purchase of a 1965  5 hp Evinrude that I decided that I better at least inspect the impeller.  When the lower unit was pulled, the impeller did not want to come off, but rotated 90 degrees when in location on the driveshaft.  By rotating it and pulling, I finally got it off.  What I found was the hub of this impeller was made of plastic instead of brass and the key had chewed up this plastic hub, allowing it to rotate inside this chewed slot.  Time for a replacement.




Copyright © 2014 -  2016 LeeRoy Wisner  All Rights Reserved


Back to Ramblings


Originated 05-29-2013, Last updated 06-11-2016
Contact the author