Replacing Water Pump Impeller in Old Mercury Outboard

I'm going to replace the rubber impeller in the water pump inside my old 1980's Mercury 90 HP outboard boat motor. This involves removing the lower unit, taking it apart a little bit, then re-installing it back on the motor. This maintenance will help insure your boat motor runs nice and cool as designed.

[@jimseefeldt2252]

Regarding whether or not the old impeller was actually bad, remember that in order to pump water, the impeller has to have a good solid seal against the interior of the pump housing. This seal is what provides the pressure needed to push the water vertically into the motor. If that seal is weak, it allows water to pass between the impeller vanes and the pump housing and the resulting reduction in pressure keeps water from travelling up the intake tube. Note that new impellers have straight vanes, while old, used impellers have bent vanes. As noted in the video, you have to kind of force the new impeller into the pump housing by spinning it and bending the vanes so it will fit. Being wedged in there like that is what provides the solid seal against the side of the pump housing. If you go back in a week and remove the impeller, the vanes will try to straighten back out. But over time, these vanes weaken and become permanently curved. At that point, they aren't really pushing against the pump housing very firmly anymore. So even if an old impeller looks OK, isn't dry rotted, etc, it can still be underperforming because it's no longer able to get that tight seal. I personally don't go more than two years without replacing at least the impeller, and each time I perform this service to my boat I also scrutinize the gaskets and plates to see whether or not they need to be replaced as well. Even if you haven't used your boat much, the impeller can dry rot, so 24 months max is my threshold.

[@YardworkWithJohn]

Yeah I'll never know what my problem actually was. It could have been the impeller, it could have been kinks in the water output hose, it could have been a super old thermostat. All three seems a little iffy but not too bad, so many all three were working together to reduce just enough water flow to give me problems. But I replaced all three (not in this video) and it runs nice and cool now! No more rowing my boat home :D

[@joewg3]

Having the same issue getting the lower unit in myself! It's the entire reason I watched the video. Now... back out to the shop. Glad it's not just me!

[@YardworkWithJohn]

Yeah it was a real pain. It took a lot longer in person than it looked in this highly edited video. You gotta just keep working it and eventually it will go in.

[@joewg3]

@@YardworkWithJohn I found that turning the flywheel worked for me! Thanks!

[@metrork1]

Possibly a ratchet strap might work while trying to push the lower end consisting of the impeller back in.

[@Gnarz1234]

Awesome video! I've been thinking about replacing my impeller myself but haven't found a video of a similar enough engine to be confident doing it. I have a 1982 Mercury 90 so this is perfect guidance. Will definitely rewatch when the time comes. Thanks!

[@YardworkWithJohn]

Put the shifter in drive/forward before you remove the lower unit! I think that will make reassembly easier. Good luck!

[@rogerbranham5083]

I have an 1989 mercury 80hp, can't find anyone who sells the impeller, where may I ask did you get yours, thanks

[@YardworkWithJohn]

Oh you can get them everywhere. You just need to google the part number. The forum on iboats.com is a great source for info like this. I believe this was the exact one I purchased but if you google that number, you can get them anywhere: www.boats.net/product/mercury/43026T2?titan_sku=43026T2

[@rogerbranham5083]

@@YardworkWithJohn Thanks!

[@ryanjames5799]

I have an 87 mercury 135 and am currently struggling with getting it to remount. So you put it in gear and then out of gear then it drive shaft seated properly? I will try that, the drive shaft just is not matching up. Thank you for the tip and great video.

[@YardworkWithJohn]

I was having a lot of trouble too. Lots of clips edited out of me trying to jam it up there and it not going. One thing people on the forums told me is that grease on TOP of the shaft will stop it from seating properly. Assuming thats not the case.... So what ultimately worked for me was what I did with the jack, I put a little bit of upward pressure on the lower unit so when I moved the throttle, it was able to suddenly side up. You could do the same thing if you had a helper. Have someone put the boat in and out of gear while you physically push it into place and see if it goes.

[@ryanjames5799]

@@YardworkWithJohnThank you very much for your reply. It turned out to be the marine grease I put in the grooves, I cleaned it out and it fit in with a little resistance. Your video was really helpful because I could not figure out what was wrong.

[@ericjump5337]

For the love of boats and all that floats...I've watched 6 videos about removing lower unit on merc outboards and absolutely none of them explain getting the shift linkage off...

[@YardworkWithJohn]

I don't think there is anything to get off. When you slide the lower unit off, the shift linkage is another smaller shaft that slides out of the top of the engine at the same time the main drive shaft does. Unless you're talking about something else?

[@ericjump5337]

@@YardworkWithJohn thank you .

[@littletoze]

Marine grease? Do you mean lithium grease? Did you have to drain the 90w from the lower unit first? Curious if you were to twist the prop a little to align the gear/shaft, if that would have made it slip in easier?

[@YardworkWithJohn]

I believe this is the grease I use: amzn.to/3UDrdK5 I did not drain the gear oil in the lower unit. And spinning the prop wouldn't help because it wasn't in gear when I removed it. If I had put it into gear first, then its possible spinning the prop may have helped.

[@metrork1]

I think the reason the lower half wouldn't go in easy is because the splines on the shaft were misaligned.

[@NicholasQuickLiveStreams]

is the prop on your boat the "Stock Prop" if it is, what pitch

[@YardworkWithJohn]

Its the prop that came with the motor when my dad traded in the 55 HP back in 1992. That's all I really know about it.

[@smeg8557]

Can I ask you what kind of grease are you using?

[@YardworkWithJohn]

I'm using a thick quicksilver marine grease, I believe this is the exact grease I'm using: amzn.to/4cDLTtT

[@smeg8557]

@@YardworkWithJohn thank you

[@smeg8557]

One more thing you don’t have to torque any of the bolts?

[@YardworkWithJohn]

You know, it's one of those things. I'm sure if you asked Mercury tech support, they'd say yes you have to torque all the bolts. But I've taken apart and reassembled enough different things in my days that for something like this, I get by just doing it by feel.

[@smeg8557]

@@YardworkWithJohn yeah I was thinking the same thing I watched a couple other videos and they didn’t torque anything either. Thanks so much for replying. I really appreciate it.