SJ23 Tech Tip D08, (Issued  2013-04-27) Bob Schimmel


Outboard Maintenance - Replace an Impeller on a Merc 7.5HP.

I get lots of questions from frustrated "DIY mechanics" about replacing the water pump impeller in their outboard.  While I can't give manufacturer specific instructions, there are many steps that are common to all outboards.  The centrifugal water pump is always installed inside the lower unit that houses the gear case.  This makes it self priming and lubricating.  So is the shift mechanism, clutch assembly (in some outboards) and obviously the propeller.  Some of the tips mentioned here are not in the service manual. 





Place outboard shift lever in neutral.

- After removing the lower unit, DO NOT rotate the shift lever so the gears stay in alignment with the shift lever when reinstalling the lower unit.


Remove the bolts that hold the lower unit (gear case & propeller) to the upper unit.

The lower unit will usually drop off now for a small outboard without a shifter.


For an outboard with a shifter, operate the shift lever to forward to separate the lower assembly about 1/2" from the upper.  This exposes the screw that connects the upper portion of the shift lever to the lower portion. 

Having removed the screw through the crack you should be able to pull the lower assembly away from the upper.  If you have a long shaft converted outboard, look for the screw above the spacer that was added to the leg. 


Remove the screws that hold the top half of the water pump housing.  Remove the cap and note the condition of the impeller vanes and which direction they point.

Place the impeller on the work bench with the vanes pointing in the same direction when it was in the pump housing. 
- Replace the impeller if the vanes are hard, cracked or missing.  You MUST install the replacement impeller in the same direction.



With the impeller removed, inspect the inside of the top and bottom sections of the water pump housing.

If the housing surfaces are smooth then they are OK to return to service.  If they are rough, you should replace them.  A rough surface can quickly wear out an impeller.  Roughness is an indication of sucking up sand in shallow water. 
- Outboard water pumps are displacement type pumps and depend on the side and end seals of the impeller to move water.  If you run this pump dry for only one turn you will likely tear off the fine seals on the sides of the vanes.


If any rubber is missing from the impeller count on it being inside the block.  It must be removed so water flow is not restricted.

Direct tap water into the water outlet nozzle of the outboard, letting water flow at full force.  Use a fine tipped nozzle on the end of a garden hose.  This back flushes the water cooling channels in the block and should remove any clumps of rubber.  On my outboard it spit out rubber particles on the driveway almost immediately.  If you can't spot them, then you should notice the water flowing more freely afterwards. 


Install the key pin that couples the impeller to the drive shaft.  Ensure the key pin is squarely seated on the drive shaft so it will slide in the slot of the impeller.

I use a dab of grease to hold the key pin in place.
- Cooking oil will protect the vanes for the first few turns of the outboard. 
- It is absolutely essential that the lower leg be immersed in water for impeller lubrication and cooling.
- Cooking oil is also useful for winter storage if you can get it in there.


Install the new impeller by rotating it into the water pump housing in the same direction it turns when the outboard runs. 

To protect the rubber vanes during installation spray them with cooking oil or apply white grease before twisting it into the pump housing.  The lubrication protects the all important side seals and tips of the vanes.  Don't tear the rubber on the sharp corners of the pump housing.
- If you have difficulty compressing the vanes, tighten a hose clamp around them.  Hold the vane over the base and nudge the impeller in.  Think ring compressor for a piston.
- If you install the vanes pointing backwards they are likely to break when the outboard starts.



Lower the pump housing cap over the base and the tighten the screws.  Be careful to align the SS plate correctly on top of the impeller.

With some outboards is it important to torque these screws to ensure the pump doesn't leak.  A leaking pump can't deliver all the water to the block.


Install the lower leg unit to the power head.  Tighten the nuts you removed in step 1. 

It is fairly easy to align the drive shaft to the power head and confirm it is installed correctly.  However, on some outboards it is difficult to also align the water delivery tube into the pump outlet.  Confirm both, don't guess.
- For some reason I have to invert my outboard to align the drive shaft and water delivery tube.  Up side down works every time.


Start the outboard with the leg in the water at least as deep as the cavitation plate.

After the outboard starts you should immediately see a strong stream of water leaving the outlet nozzle. 
- If there is no stream of water, I hate to say this but you "screwed up."  Not nice to say but either the water delivery tube is not inside the water pump (step 10) or the key pin is not seated correctly (step 7).  I learned this lesson the first time I replaced an impeller. 

MANDATORY - The stream of water leaving a running outboard MUST be checked each time you start it.  If there is no flow, shut the outboard off and determine the problem.  If the flow shows signs of weakening, it is time to replace the impeller.  Some manufactures recommend replacing it every 2 years, regardless.  If you are powering the boat for a long distance it is a good idea to pay attention to the outboard temperature (touch the bottom of the power head) and check the water stream every half hour (stick your hand in the stream to feel the temperature).  A hot outboard without water in the cooling jacket has a hollow sound (don't ask!) that just isn't healthy.  Shut it off before it burns out.  Check the water inlet for blockage.  Hopefully that's all that is wrong.  I bevelled the inlet holes of Panache's Merc 75 a tiny bit to improve the water flow.  Good luck.
2021 - My Merc 75 had reduced water flow for several years but it was still sufficient to cool the block.  I watched it closely but it was time to do a repair.  The old impeller had 2 cracked vanes so I replaced it.

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