How to Replace a Yamaha 90HP Outboard Water Pump

Knowing how to replace the water pump is an essential skill every Yamaha outboard engine owner should know. 

Water pump impellers are prone to wearing out pretty quickly, and should be inspected every 100 hours or once a year. Replacing your Yamaha 90HP outboard's impeller is inexpensive, and gives you at least a year of not worrying too much about the water pump failing on you. 

Tools and Parts Needed - Yamaha 90HP Outboard Water Pump Replacement


  • Hammer
  • Punch
  • 3/8th ratchet
  • Ratchet extension bar
  • Screwdriver or pry bar
  • 12mm socket (deep)
  • 14mm socket (regular)

These parts are for a 1999 Yamaha B90TLRX outboard model. Water pump repair kits and impellers for all other Yamaha outboard motors are available here on 

Yamaha outboard motor water pump repair

NOTE: We recommend you buy the complete Yamaha water pump repair kit, as it includes the various gaskets, O-rings and other parts that usually need replacing when you repair the water pump. It's much cheaper to buy the complete kit than it is to buy the impeller, gaskets and O-rings separately. If your Yamaha 90HP water pump has been neglected for some time, it may also be necessary to replace one or both halves of the water pump housing.

Replacing a 90HP Outboard Yamaha Water Pump 

Before starting this job, see the Yamaha B90TLRX outboard lower casing drive parts diagram for additional reference.

Step 1. Remove the trim tab by prying off the cap from the foot of the upper casing and taking off the 12mm trim tab bolt beneath it.

PRO TIP: Before removing the trim tab, draw a mark across the trim tab and lower casing so you can reinstall it in precisely the same position.

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Step 2. Remove the 12mm mounting bolt hidden under the trim tab, then remove three of the four remaining 12mm mounting bolts (two on each side) that hold the lower casing to the upper casing.

NOTE: Do not remove the fourth mounting bolt yet, or else the lower casing will fall off.

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Step 3. Unclip the pipe from the water pickup tube at the front of the upper casing that feeds down into the lower casing.

Step 4. Slowly unscrew the last 12mm mounting bolt holding the upper and lower casings together while supporting the lower casing, and slide the lower casing away from the upper casing. Place the lower unit on a stand or workbench.

NOTE: Be careful not to damage the driveshaft or water pickup tube as you remove the lower casing. 

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Step 5. Remove the four 12mm bolts from the water pump assembly, then take off the upper water pump housing. Inspect the inside of the housing for wear or damage, and discard it for a new one if necessary.

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Step 6. Remove the metal cartridge insert by pulling it upwards while rotating the driveshaft clockwise.

Step 7. Remove the impeller by inserting a flathead screwdriver or pry bar beneath it, and carefully levering the impeller up and off the driveshaft. Inspect the impeller for wear, tears, splits or broken fins.

PRO TIP: You can reuse the impeller if it's in good condition, but we highly recommend replacing it with a brand new one regardless of what condition it's in.

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Step 8. Place a punch at one end of the woodruff key and strike the punch with a hammer to rotate the woodruff key out from the driveshaft.

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Step 9. Slide the metal plate and the paper gasket up and off the driveshaft to access the lower water pump housing. Inspect the lower housing for cracks or excessive corrosion, and discard it for a new one if necessary. 

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Step 10. Install a new gasket over the lower water pump housing, and install the metal plate.

NOTE: The plate has small ridges that must be placed downward into the lower housing's channels.

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Step 11. Reinstall the woodruff key by placing it into its slot and hammering it back into place.

NOTE: Be careful not to damage the paper gasket on the lower water pump housing, and make sure the woodruff key is parallel with the driveshaft once it's back in place.

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Step 12. Grease the driveshaft and the inner core of the impeller with marine grease, and slide the new water pump impeller down over the driveshaft.

NOTE: Make sure the notch in the underside of the impeller is seated over the woodruff key as you slide it down against the metal plate.

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Step 13. Grease the inside of the metal cartridge insert. Place the cartridge on top of the impeller and rotate the driveshaft clockwise while pressing the cartridge down over the impeller.

NOTE: It is very important that you rotate the driveshaft CLOCKWISE so the impeller fins are turned in the correct direction within the cartridge.

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Step 14. Reinstall the upper water pump housing and the four 12mm bolts.

NOTE: Do not over-tighten the bolts, as doing so will damage or break the plastic upper housing.

Step 15. Apply marine grease to the splines at the top of the driveshaft and the splines on the shift shaft that's next to the water pump assembly.

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Step 16. Reattach the lower casing to the upper casing. As you slide the lower unit up, you'll need to make sure the driveshaft splines are correctly lined up. Next, ensure the water pickup tube has cleared the upper casing, and then ensure the shift shaft splines have engaged.

PRO TIP: Have one of the 12mm mounting bolts ready, so you can thread it into place to hold the lower casing once you have the lower unit lined up and seated back up on the upper casing.

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Step 17. Quickly reinstall the remaining three mounting bolts to minimize the strain on the first bolt, and tighten all four bolts to 29 foot-pounds with a torque wrench.

Step 18. Reinstall the 12mm mounting bolt under the trim tab, and torque it to 29 foot-pounds.

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Step 19. Reinstall the trim tab (making sure to line up the mark you made earlier if you followed our PRO TIP in Step 1) and replace the plastic cap at the top of the lower casing.

Step 20. Reattach the pipe to the water pickup tube at the front of the upper casing and you're done!



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