Replacing a Water Pump or Impeller on Yamaha T8  8HP Outboard Motor





Peculiarity of This Motor/Water Pump;   In owning this motor for 8 years I have found that IF you let it set for an extended period of time (possibly 6 months) that when you start it back up using muffs, that your water pump may not pump water out the tell-tale hole.  No blockage in this outlet either.   I have torn mine down only to find the impeller is still very much intact and funtionable.  It just doesnt pump water through the powerhead.


I have inspected the old impeller and reinstalled it.  I will also grease it in the cup when re-installing.  When starting the motor after re-installing the old impeller, it pumps water like it should.    My conclusion is for some reason this motor becomes air locked if allowed to set for a period of time.


My solution was to run a garden hose directly to  the flush hose fitting, turn the water on and start the motor.  Cover the tell-tale hole with your finger so that you divert all the water into the powerhead.  Let it run for a few minutes, shut it off and reconnect the hose to the muffs.    BINGO  -- it now is pumping water.   It seems that by backflushing under pressure while running, that this removes any air lock.


I wonder just how many of these motors are taken to a maring mechanic in this situation and a new impeller is installed with nothing wrong with the old one??


Removal Procedure ;   This repair will be the same for most of the newer the 9.9hp motors as well, as the only real difference here is the size of the pistons in the powerhead.


If you have to do this repair, here is the procedure for replacing the water pump or impeller on these motors.  This will probably also be very similar to about all of the other small Yamaha motors of this vintage.


There are 2 bolts on each outer side of the lower unit that need to be removed.  Also, in front and in plain view between the exhaust housing and the lower unit you will see a threaded haft with what appears to be a double nut on this shaft.  These nuts are in reality a removable coupler and shift adjustment with the lower thinner nut being a jam nut holding the upper hex shaft in place, which is the lower end of the shifting rod from the motor.  The jam nut needs to be at the precise location on the threaded lower unit shifting rod to time the transmission gears for forward, neutral and reverse. 


Once you get the outer 4 bolts loose (using a 12MM wrench) the lower unit may drop down slightly on the rear, but will be still held up by this connected shifting rod on the front.  Shift the motor into reverse which will lower the upper hex so you can access it with a wrench.  Using two 10MM wrenches, loosen the lower jam nut but do not turn it other than that just loosening it.  Then raise the upper hex shaft by rotating it counter-clockwise, threading it farther on the shaft it is attached to.  This will need to be moved up about 5/16" or so until it becomes unscrewed off the lower threaded shaft.


Now count the number of exposed threads on the shaft that are BELOW the jam nut to the upper end of the shaft.  Write this down as you will need this information later.  Now you will need to unscrew the jam nut off the shaft so the shaft will slide out of the mating hole in the lower part of the exhaust housing.  On my motor, the space of threads showing below this thinner jam nut is almost the same spacing as the nut itself.


Pull the lower unit along with the driveshaft out the bottom, and free of the rest of the motor.


Now using a 12MM wrench, preferably a ratchet and socket, remove the 4 bolts on the top of the water pump housing.  If the housing does not wont to be pulled off, you may have to rap it with a hammer handle, enough to jar it loose from the adapter plate it sits on.  It will not rotate because of the 2 alignment pins at the rear as seen in the photo below.


Usually the impeller will stick to the driveshaft and you may have to wedge a screwdriver blade under it, forcing it up enough to loosen it allowing you to then slide it up and off the driveshaft.  If the driveshaft has any degree of debris it, you may have to clean that off before you can pull the impeller up and off.


Water pump housing loose & lifted off the impeller


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.  .


Worn water pump cup & lower wear plate



Reassembly Procedure ;  The impeller drive key will usually stay in it's groove in the driveshaft when you pull the impeller up and off.  If you are replacing the bottom stainless wear plate, the key will need to be removed.  A Vise Grip works great here.


In reassembly, clean everything up.  At this point, there is a right side of the impeller as the keyway does not go all the way through the impeller hub.  Therefore you need to place the impeller inside the stainless steel housing with the non full keyway slot in the final UP position so the open slot will slide over the key in the shaft.  Lubricate the stainless pump cup and the impeller with marine wheel bearing grease (to ensure it does not seize to the cup and ruin the impeller before water starts pumping, providing it's own lubrication), get the impeller in the cup by rotating it slightly in a RH rotation looking from the topside (remember you will be looking at it from the bottom side.  Now the blades will be bent.  Be sure all are bent the same direction.  Remember the direction of the impeller vanes (if any were left on) when you initially lifted the housing off.  If you did not notice, they were all missing or you forgot, then remember that all of these motors rotate clockwise.  But AGAIN since you will be looking at the bottom side of the water pump, you will be seeing it backwards. 


When assembled in the pump housing and bolted onto the lower unit, the driveshaft and impeller rotates clockwise as does the motor.  Since the pump housing is smaller than the impeller, the vane tips need to be facing in a counter-clockwise position as they are FOLLOWING the rotation of the driveshaft which forces the vane tips backwards.



Birdseye view  looking down from top


The motor has to be submerged in water above the level of the water pump.  The impeller pulls the water in the pump unit and then pumps it up into the engine.


If you have a problem understanding, and your motor is a manual start motor, take the cowling off and pull the rope, noting which way the flywheel is rotating.  Translate this into the direction the driveshaft will be rotating.  Since this pump housing's stainless cup is smaller and not in the center, the impeller vanes will need to be collapsed as a trailing edge opposite the direction the driveshaft will be rotating.   Again be sure all are bent the same direction, however some will be bent more than others.


Once you have the impeller inside the housing, I have found it easier to rotate the impeller so that the keyway is pointing directly to the rear of the housing.  The rear is the end where the rubber water outlet tube slides into.  You may have to clean and slightly oil the lower part of the driveshaft for easier sliding of the now installed impeller (which is in the housing) as you push the housing down the driveshaft.  Before you get this far though, you will need to rotate the prop so that the impeller/driveshaft key is also aligned at the rear of the water pump baseplate/adapter plate.  As you slide the housing down, try to align the key slot with the shaft key.  This may take a few tries, before you get it to slide down and  started onto the key, then bottom out on the stainless steel base plate.  If you get things to where you think it is down, but it will not bottom out by about 1/8", you have the pump housing on backwards as the 2 alignment pegs on the rear of the base have to align with mating holes in the pump housing.


Now inspect the 4 stainless bolts that hold this housing onto the lower unit.  If they have any oxidation/corrosion crud on the threads, I suggest you wire wheel the threads clean and smear some anti-seize compound on the threads.  Better to do this now than down the road if you have to do this again and those bolts become corroded and break off during the next repair.  Tighten these bolts down.  Now check the water outlet rubber grommet to see that it is locked in place by the small round rubber "ears" that protrude on each side and lock into round holes in the top of the housing.  The location of these are shown by the red arrow in the photo below.  If they have become dislodged, using a screwdriver by going inside the grommet and a simple push the grommet down enough to allow the ears to pop into place has worked for me.


Water pump assembled  with arrow pointing to grommet protrusions 



I like to smear some boat trailer waterproof chassis grease on the upper driveshaft splines before I slide everything back together.  Again, look ahead so the next guy (which may be YOU)does not have a problem with the driveshaft stuck in the lower end of the crankshaft.  


Slide your repaired lower unit's driveshaft up inside the exhaust housing.  First you will need a flashlight so you can align the water tube into the rubber grommet of the water pump.  The tube is cut at an angle to facilitate a somewhat blind assembly.  You will have to align the driveshaft to the crankshaft splines by feel, if they don't readily slide together, you may have to rotate the prop slightly, however on my motor, there was never any problem getting these shaft splines to engage the mating ones in the cranksahft.  Now slide this whole lower unit up and align the shafting rod into it's hole in the exhaust housing.  Move things up and hold it there so you can start the jam nut onto the threaded shaft.  You may wont to start at least 2 of the outer 4 bolts holding the lower unit to the exhaust housing to facilitate holding it there as you connect the shifting rod coupler, unless you have a helper.  


First, thread the jam nut onto the upper shifting rod.  Thread it down to the number of threads you counted that were protruding on the disassembly.  In my case, my motor had 8 threads exposed.  Now slide the unit up slightly and start threading the lower part of this hex shifting rod shaft onto and down until you come in contact with the jam nut.  Try not to dislodge the jam nut from your preset location.  Using 2 wrenches, tighten the 2 nuts together, locking the shaft in place.  When finished it should look like the coupler/adjusting nuts in the photo below.



Shifting rod coupler nuts, the top is a LOOONG nut threaded onto the lower end of the upper shifting rod & the lower just a jam nut securing it in place


Clean (wire-wheel the threads and grease with anti-seize) Install the rest of the outer bolts and tighten them all down, and you are finished.  This should take less than an hour to complete from start to finish if you have all the parts on hand before you tear it apart.


As a test, hook your motor up to a set of muffs, turn the garden hose water on and then start the motor.  If it is not "peeing" check the Pee hole for a blockage by poking a wire or heavy monofilament line up into this hole to dislodge this debris.  These motors do produce a very generous flow of water here.


Another thing to look at yearly is the motor's thermostat.   These things are not cheap, (like over $40) but are curtail for proper motor function.  They are located on top of the block just behind the flywheel, but the timing gear and flywheel cover needs to be removed to access the thermostat.  Two bolts removes it's cover.  Take it out and boil it in a pan of water.  It should open at about 165 degrees.  If it does not move by the time the water boils, time for a new one.  Aftermarket ones made by Sierra are about $10 cheaper.


Panic After Replacement if it Does Not Pump ;  If you are using it on muffs or even on the boat, if this happens, shut it down ASAP.  There is a garden water hose fitting on the right side of the motor designed for flushing.  Hook up your garden hose to this fitting as if you were flushing, and start the motor, BUT put your finger over the Pee hole, plugging it.  This diverts all the water backwards thru the cooling system.  After it has ran for a few minutes and warmed up, let go of the finger, it will Pee because this flush fitting is Teed into the Pee hose.  Shut it down and replace the muffs on the lower unit water intake and restart the motor.  99.9% of the time it will now be functioning properly.  My explanation for this is that more than likely the motor has sat for a time, (like from fall thru the winter and into spring) and any water inside will have dried up, and with the thermostat being cold will be closed therefore possibly creating an internal air-lock when you try to restart it.




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Originated 03-02-2013, Last updated 07-19-2018
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