This may be a stupid question but...

[Buh-Bye]

Do you have to drain the lower unit oil before replacing an impeller? I have a Mercruiser Alpha One and the repair manual has instructions for both, but does not state that you have to drain the lower unit prior to replacing the impeller. I have not done either before but believe I am handy enough to handle both. I figure I would drain the lower unit, replace the impeller, put it back together and refill with drive oil.

Also, how often does one replace the impeller? I have had the boat one season (and had other boats prior to this one) and would like to change it for the sake of knowing the last time it was changed and is good to go. The motor runs fine on muffs and cycles water through the engine no problem. Temp guage holds steady at a little less than 170 degrees and so makes me wonder if I should even replace it if it seems to be in well working order already.

Guess that was multiple questions but would appreciate any insight you folks might have. Thanks in advance.

 

[thumpar]

Re: This may be a stupid question but...

Yes, drain the drive. If you don't you will have a big mess. I replaced the impeller in my Alpha every other year since we only put about 15 hours a year on it and boat in clean freshwater. I will probably do the for the sea water pump on the new boat.

 

[Watermann]

Re: This may be a stupid question but...

I do the same as thumpar, drain the gear oil first and every other year unless you get a new to you used boat then order the water pump kit for it. If you open it up and the impeller housing is scored up then you'll be waiting for more parts to arrive. After that you could just replace the impeller every other year although some do the kit every time. I would also recommend getting new gaskets for the drain and vent plugs.

Sounds like she's running good for you right now but who knows when one of the vanes will let go on the old impeller. Piece of mind is always better than over heating and huge repair bills.

 

[bspeth]

Re: This may be a stupid question but...

pressure test it while its drained,and make sure it shifts right before filling it up.

 

[airshot]

Re: This may be a stupid question but...

Running on the muffs is not a good test as the water pressure from your hose will circulate the water thru the engine and give you a false impression that the impellor is doing its job. Only real way to tell is by sitting in the water and running the engine, but as others have posted you need to know what you have in there to start with. I replaced mine after two seasons and found the old looked just like the new one so I am going to go three seasons before change but I do watch the heat gage closely, and have an auxiliary motor just in case.

 

[Volphin]

Re: This may be a stupid question but...

You can also test the impeller in the water by pulling a manifold hose and starting the engine for a moment. You'll see it pumping.

 

[haulnazz15]

Re: This may be a stupid question but...

I change mine every 3-4 years, but that's my own prerogative. I only put on 25 hours or so per year at the most, and every impeller I have pulled out over the past ~20 years has looked just fine w/no cracks.

 

[Buh-Bye]

Re: This may be a stupid question but...

Thanks for the input. I'll probably do a water test at the ramp (good point about running on muffs airshot) or just go ahead and change it. I helped my neighbor do his impeller on an 89' Regal 21 footer last fall...looked like it had never been changed. I'm probably more of a 2-3 year rotation depending on the amount of boating I can get in on the season.

 

[oldjeep]

Re: This may be a stupid question but...

I change mine every 3-4 years, but that's my own prerogative. I only put on 25 hours or so per year at the most, and every impeller I have pulled out over the past ~20 years has looked just fine w/no cracks.

x2. In 9 years I replaced mine twice and they both looked new. I did pull it out every couple years to look at it. (Replaced it a 3rd time the week before I sold it).

As long as you are not running it dry or feeding sand through it the Alpha impellers really seem to last.

In contrast, the ones that they use in my current inboard look like they need to be replaced every year or 2, they really distort the crap out of them in the Johnson pumps.

 

[haulnazz15]

Re: This may be a stupid question but...

I'd be a lot less hesitant to change it if it was designed like the Volvos or an engine-mounted pump, lol.

 

[oldjeep]

Re: This may be a stupid question but...

I'd be a lot less hesitant to change it if it was designed like the Volvos or an engine-mounted pump, lol.

Yup, that is the only thing I missed about my old OMC Cobra. You could change the impeller in 5 minutes with the boat sitting in the water.

The engine mounted pump in a V-drive is still kind of a pain in the rear, you have to do your yoga stretching before attempting.

 

[Buh-Bye]

Re: This may be a stupid question but...

Well, I changed the impeller and lower unit oil yesterday. Probably took me longer than it should have as I had to discover a couple "hidden" bolts and had a tougher time than expected trying to lift the lower unit back into place. The old impeller looked in good shape (as did most of the seals - except the drive shaft seal was starting to crack). Glad I got it done and no longer have to worry about what is in there. Now if I can only convince my wife that boating with our two small children this summer won't be that bad...

Thanks for the comments and suggestions from everyone...def some good input and much appreciated.

 

[ktbarrentine]

Re: This may be a stupid question but...

Good job attacking the impeller replacement. You mention you've had the boat one season.... does that mean that it is dealer-fresh +1yr, or is it an older model that is "new-to-you"? (hint...should always give us year/SN of drive/engine and operating history for the best advice).

If it is new to you, but has some years on it, it is always good practice to replace the impeller as soon as it is yours. Plus, highly recommended to visit the adults only stickys at the top of this forum for sage words of advice and information on what-to-dos and how-to-dos, plus excellent video links from the "masters" on what you should do seasonally as preventive maintenance, and how to get your rig ready for new boating season. Things like, when you have your drive off the boat for impeller replacement and air test (you said you only took off the lower. Did you just take off the lower, or the whole outdrive?). When the drive is off, you do routine maintenance and inspections on the drive bellows, exhaust bellows, shift linkage, U-joints, and very importantly check engine-drive alignment before reinstalling the outdrive.

How does all that look on yours? (again, if dealer-fresh, you should be ok till next season).

You also didnt mention salt or fresh water operation. This also affects the type of maintenance actions slightly (anodes and such)

You mentioned drive shaft seal "starting to crack" just how bad, and did you replace it? Which of the two drive shaft seals is it (two seals seal the drive shaft area where the lower drive shaft penetrates the upper drive cavity. Older Alpha-ones differ in this seal arrangement versus newer (91+) alpha-one genII versions, so replacement complexity is different.

Did you pressure test before putting oil back in as recommended?

Looking forward to hearing back from you and happy boating! Should be near 100 in Phoenix today and I hear Lake Pleasant calling us to come out and join the fun!