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DYI Impellar Changeover


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Just did my own water pump changeover. I have a 135 Merc. Optimax. Its an 05' and last year somewhere between 1500 and 2000 rpm, I'd get an over heat buzzer. It would go away at idle and WOT. Figured I was due. $50 bucks and three hours of Van Halen, AC/DC and two buds its ready for a field trial if this white $#@! ever goes away. If yours is three years old or more the flutes can take a set and lose efficiency.

If you dare.....Here's about how it went.

  • Put the engine in forward.
  • Tilt the engine a high as you can.
  • Remove the prop and rear anode.
  • Remove the bolt hidden up under the anode from the bottom side.
  • Remove the two nuts under the swash plate just forward of the prop.
  • Remove the two nuts forward of them and above the swash plate.
  • Remove the single nut on the leading edge of the lower unit.
  • Carefully use a rubber mallet to loosen the lower unit. Watch for the gap as it comes apart and keep it even from front to back as it slides off.
  • When you get it seperated at about three inches, theres an 1/8 water tube connected with a push lock fitting. Remove it by pressing on the brass lock on top of the fitting while gently pulling out the tube. Now your free.
  • Carefully remove the lower unit and place it on a bench vise that you previously placed a towel between the jaws.
  • Remove the three nuts on top of the housing, plus one screw.
  • Slide the housing up and off the shaft.
  • There is a key which engages the inside diameter of the impellar, it may fall off the drive shaft. No problem, a little grease will help keep it in place when you begin to re-assemble.
  • Remove the gasket, stainless plate and bottom gasket by sliding them up the shaft and off the shaft. Keep them together as reference.
  • Clean all flat sealing surfaces of the lower units housing base.
  • Assemble the new gaskets in the same manner as the old and feed them over the drive shaft.
  • Wipe some grease inside the housing wall and insert the new impellar being sure the flutes are going in the same direction as the old. Be sure to rotate them in the same direction the old flutes. Once full installed, clock the impellar to match the keyway position of the old one to engage the key on the shaft.
  • Slide it back down the shaft.
  • Reinstall the nuts and bolt. Don't over torque the bolts.
  • Grease the spline end of the drive shaft and the spline end of the gear shift shaft in front. Grease all of the bolt threads in the lower unit while your at it.
  • Put the new plastic tube into the housing just like the old.
  • Carefully slide the unit back into the upper unit of the outdrive. At about a three inch gap, push the little water tube into the push lock fitting.
  • At about a two inch gap, align the main copper water tube with the plastic tube.
  • At about a one inch gap, grab the upper units shift tube and align it with the lower units spline.
  • If the Gods are smiling the last inch should go right in. If not, you may need to grab the prop shaft and rotate it while you push the rest of the way to get it to align with the power head. You may need to move your shift lever to get the drive tube to engage the small spline shaft in the lower unit.
  • Close the gap until you can get that forward nut and washer installed with your finger.
  • Reinstall all of the nuts in reverse order.
  • If all goes well, you just saved several hundred bucks.

Like any DIY project, you still run the risk of making a mistake which could end up costing you more than the original service fee from a mechanic. This will result in a face to face with the trained mechanic. You'll have to endure his smile and the head shake because tried this yourself and didn't hire a proffesional in the first place.

I've done three of these, both outboard and I/O, they are virtually the same. So far, I haven't had any problems. I'm not a trained mechanic, I don't play one on TV and I didn't stay at a Holiday Inn Express.

Good Luck!

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Yep, always a good plan to change the impeller. Mine will get changed next Winter. I do mine every 3 years. Cheap insurance. Boat that get a lot of shallow water use and churn a lot of dirt should probably change impellers yearly or at least every other year. I use my boat a lot, but I'm most always in deep water. It's not rocket science but like take six points out there is a procedure that needs to be followed.:)

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