By Steven Bull
Like with homes, there’s always a project to do on the boat. Determining which jobs can be tackled by the DIYer and which to leave to the pros can be obvious: repairing a cracked engine block? Leave that to the pros. Changing the oil? A simple DIY. Then there are projects that fall somewhere in between. They are feasible to do but unlikely for the average boater.
Replacing the impeller on the lower unit of a 5.0-L Mercruiser stern drive is one such project.
It’s complicated, but it’s not out-of-hand confusing. It requires some specific knowledge and at least one “trick” to know but nothing you can’t find online. No super special or rare tools are required, though one piece of equipment will make your life much easier.
Most people leave this to their marina’s service department or their local boat mechanic. So, why keep reading? Well, there are times you may not have that option. You could find yourself somewhere remote with no mechanic nearby or the service shop might just not be able to fit you in before your family reunion weekend.
A replacement impeller is something that you should have on hand in your emergency repair kit, along with spark plugs and ignition wires. When you need to replace it you don’t want to have to wait for parts!
This job needs to be tackled out of the water, so getting it on the trailer is first. Step two is putting it in gear. This step is that trick. Why do this? It stops the prop from spinning in reverse and it will only come off this way. You can tell it’s in gear (not turned on!!!) if you hear clicking when you turn it and can’t turn it in reverse.
Then you loosen the bolts holding the lower unit on. Once all are loosened and you can “crack” it off (sometimes it’s quite stuck), you can remove the bolts entirely and remove the heavy lower unit.
Ideally, you should have a stand to keep the lower unit upright while you work on it. If you don’t, borrow (or make) one. Though it’s not the end of the world if you don’t, it just saves you from working bent over.
This is a good time to answer the “when I should change my impeller” question. The exact answer depends on where you boat, but a good rule of thumb is every couple of years. If you are in shallow, silty, or sandy water you’re likely to suck up a lot of debris that will chew away at the impeller. If you’re in deep, rocky-bottom water, that’s less of a concern and it will last longer. Either way, do NOT wait until a catastrophic failure — that’s a much bigger repair.
The impeller is within a small housing and you’ll see in the Quicksilver kit. You will see all the same items, so you can do a double-check of what to remove.
Slide the housing off. The impeller will likely be stuck inside it which you’ll need to get out. There’s also a plate to remove and gaskets as well.
Getting the new impeller into the housing is a bit of a challenge. It’s stiff rubber that needs to be twisted to get in, but a little elbow grease does the trick.
Replace everything in the exact opposite order. A good way to remember is to disassemble methodically and lay the old pieces in a line somewhere for a quick visual check.
Add sealing compound to the gaskets and plate for a better seal. And don’t forget the little tab, or key, that needs to go back onto the cut out on the shaft so the impeller will slide back down and spin around as required.
Once you have all the pieces back on, there’s one more step before tackling the reconnection of the lower unit to the upper.
There’s a small black, plastic tube — the coupler — and inside you’ll see a couple of round seals. Gently use a flat edge screwdriver to pry them out and put the new ones from the kit in their place. Replace these even if they are undamaged. You’ve got the new ones and wear and tear isn’t always visible to the naked eye.
Putting the lower unit back on will likely require some help. It’s awkward, a little heavy and you have to line up the drive shaft and coupler and bolt holes all while trying out some new expletives.
Once you have it all in place make sure you tighten the bolts down with a torque wrench because you want to make sure this seal is as tight as it should be.
It’s not the easiest job, so budget half a day for it—make it a full day if it’s your first time. You can make it a DIY, though after doing it myself for the first time, I’m sticking with my service department for the next one!