Taking Care of Your Mercury 2-Stroke Pt. 3: Water Pump Impeller

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This will be a multiple part series about how to take care of your aging early model 2-stroke outboard. Check back to further articles on other yearly maintenance items you can easily do yourself.

For this part of the Mercury and other two stroke outboard maintenance series we’ll be covering how to replace and service your water pump and impeller inside the lower unit of your 2-stroke Mercury. Mostly all 2-stroke outboards from 9-50HP will have an extremely similar method of servicing the water pump with some slight variation through different manufacturers. When I had a Yamaha 50HP 2-stroke for a brief period a few years ago I followed this same procedure and was able to service the impeller with ease.

What You’ll Need:

  • Mercury water pump repair kit or impeller kit (or for applicable to your outboard, see your users manual)
  • Wrench set or ratcheting socket set
  • Teflon grease (I recommend the Mercury/Quicksilver brand even if you don’t have one)

The DIY:

Step 1: Stare at your lower unit. 

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Step 2: Remove the cavitation plate anode above the propeller but below the cavitation plate with your ratcheting wrench.

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Step 3: Remove the nut inside the anode housing on the underside of the cavitation plate. It’s way up in there on a welded-on bolt.

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Step 4: Loosen all four of your lower unit main bolts that connect the lower unit to the middle shaft but only remove two of them. Leave two of them threaded in so the lower unit doesn’t drop free!

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Step 5: Remove the shift linkage double-sided nut from the rear corner of the lower unit. You can see this outside of the housing below the rear shock bushings.

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Step 6: While supporting the bottom of the lower unit remove the two remaining lower unit main bolts that you left threaded in and proceed to wiggle the lower unit free from the middle section it was secured too. You may need a rubber mallet or dead blow hammer if you have never done this before, to knock it loose.

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Ta-Da! The lower unit is now removed!

Step 7: Prop the lower unit up on a shop bench or pieces of wood (or anything you can keep it upright with, really).

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Step 8: Remove the four bolts bolding the water pump impeller housing onto the lower unit.

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Step 9: Slowly remove the top impeller housing by wiggling it upwards. Be careful not to lose any parts! Once it’s off you will see the impeller, two plastic washers and this tiny metal key sandwiched between the impeller and the shaft. Do not lose this ‘key’!

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Step 10: Remove the impeller and clean the area around the water pump impeller housing and base plate.

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Step 11: Reinstall the lower impeller housing plate along with two gaskets from your pump repair kit. I like to apply some teflon grease here to the bottom plate to keep the impeller moving smoothly throughout the year.

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Step 12: Re-install the impeller with two plastic washers on each side. Use caution to install the ‘key’ (shown above) in the flat slow on the main gear shaft while you press the impeller down on it at the matching notched out area.

Step 13: Re-install the impeller housing top plate and press it down onto the impeller by turning the shaft clockwise. This will properly seat the impeller as you press down the housing and make sure the fins are in the proper orientation for water pressure and flow.

Step 14: Re-install all bolts and everything above in reverse. Be sure to apply teflon grease to all bolts before re-installation to ensure that the next time you do this service nothing will be corroded or seized into the outboard housing threads. Another important note: when re-installing the lower unit be sure that the linkage shaft (the small shaft towards the rear corner which was screwed into the double-nut) is in NEUTRAL position. You can actuate the linkage by hand up and down and you can feel the forward, neutral, and rear positions. Place it into neutral before re-screwing it into the double-sided nut.

That’s it! Run your outboard on the hose and it should be good to go after everything is re-bolted on.

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