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Water in lower unit

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Mark010

Member
Hi, when I drained the lower unit oil this fall on my Honda bf40 outboard, the oil was very milky. I want to stop what must be water getting into the lower unit. Is it worth the trouble of finding the leak, or is it easier to just replace all the gaskets that could be causing the problem?
Thanks in advance for any thoughts.
 
Hi Mark,

This is just my opinion. I have never worked on a BF40. Because of age and health, I haven't done this work for a few years now. But, much of it hasn't really changed for a long time.
So, others may have different
opInions and suggestions.

While this problem seems pretty straightforward at first, it's good to know a few things that might not seem readily apparent if you haven't done this work before.

Minor leaks:
First off is that, if there's water in the gear oil, it most likely came in when the outboard was at rest and the lower end was cool. Because, as the outboard is run, the gearcase heats up and pressure builds inside. A very minor leak might then be seen as oil escaping the case or an oil slick or sheen on the water or....no leak at all.

But, as the lower end cools, the pressure inside the case disappates and, if the temperature is low enough, a slight negative pressure or vacuum can develop that "pulls in" the water.

So, an outboard that doesn't seem to be leaking can still have water in the gear oil.

There are several points where water can enter....6 actually. The mainshaft, the shift shaft, the propshaft, the propshaft holder and the two case plugs. Water might also enter via a crack in the housing.

It's always been my practice to remove, drain and pressure test the gearcase to find the leak because there could be more than one but I don't really like disturbing a seal that isn't leaking unless the outboard is pretty old.....15 + years....since many will seal way beyond that time.

On the other hand, leaving a propshaft holder in place for years and years might result in it or the mounting bolts siezeing
So, it's definitely a judgment call as to how to alleviate something like that from happening.

The most frequent area of leakage in most outboards I've ever worked on is the propshaft seal or seals. The most likely reason for early or frequent propshaft seal failure is fishing line wrap. Monofiliment line is the #1 propshaft seal killer that I know of.

To simply answer your question, yes, in my opinion, it is absolutely worth the trouble to find the leak.

However, pinpointing leaks and, making sure the unit is completely sealed after repairs requires a bit more than simply applying some regulated air pressure.

For example, the propshaft and mainshaft should be turned during the test and the shift shaft should be exercised. This is especially important after any seal replacement.

I know many will say that pressure testing is "good enuff" but I also vacuum test after repairs.

I hope that you find this helpful.

Good luck.
 
Hi jgmo,

Sorry for the radio silence on my end. I planned to work on this lower unit back in the fall. But Life got in the way. Now I'm hustling to get it repaired so I can put my pontoon in the lake.

So far I found one leak in the lower unit. It's the boot on the shift shaft. Still checking main shaft.

I hope I can still contact you with questions as I work to get this done.

Again, my apologies for not responding in a more timely way.

Mark
 
Hey Mark!
No worries mate! I have a good working knowledge of "life got in the way" myself so you are just putting me in better company.

I'm always standing by ready to jump in and try even if I often miss the....well....

.....Mark! :)

..
 
Hi jgmo,

I found the leaks and am refilling the lower unit with oil. I thought it would be easier to put the oil in before installing it on the rest of the engine. Also, I somewhere about pausing to let air bubbles rise out of the oil. And, I read somewhere that one should tip the motor down or tip it up when the oil is at the top screw hole to make sure the oil reaches all spaces. Then they say you should be able to add a bit more oil.

I'm starting to wonder if any of that advice is accurate. What are your thoughts?

Thanks,
Mark
 
Well, everyone will have their way to do it differently but my method has always been like this:

With the outboard in the vertical down position, fill through the drain hole (bottom) with fill plug (top) removed. I used to use the lube bottle with the tapered nozzle cut off and just press it into the drain hole tightly. I ultimately went to a pump bottle with a tapered rubber tip on the hose that I fashioned myself but the bottle always worked ok for me too.

When it's full, install and tighten the fill plug and then quickly remove the bottle or hose and install the drain plug.

If you have no leaks down low, the fluid level should stay pretty much the same... much like filling a drinking straw and then putting your finger over the top. It might leak a few drops but it's a fairly reliable method. Just be ready for some mess doing it this way.

Wait a few minutes then exercise the tilt up and down.

Then, wait a few more minutes and check the level. Top off as needed.

If you use a tank (trashcan) for flushing, you can run it in gear afterwards watching for any sheen in the water that might indicate a leak.

I would check the fluid level once more and see if the level has changed significantly.. if still full, you're done.

I've been disabled for some time now and haven't done this in a good while so there might be a modern "hack" that works better.

Good luck.
 
Forgot to say that you have to squeeze the bottle (that and capacity is why I went to a pump set up) to force the fluid in. You would undoubtedly figure that out on your own but just covering the bases.

A helper nearby to hold their finger over the fill (upper) hole while you switch bottles or put the drain plug in is always a plus too.
 
Very helpful thoughts. Thank you! I particularly like the idea of re-checking the oil level and topping up if necessary.
Mark010
 
Hi All,

I drained my lower unit for the winter. This is the second fall in a row that the lower unit oil looked very milky. The engine is a honda bf40a outboard 40 hp. My guess is it is about 10+ years old. Last year, I found the leak myself. It was at the boot on the shifter lever where it comes out of the lower unit, and I replaced that boot. I re-tested and found no more leaks.

This time, I removed the prop and checked for fishing line on the prop shaft and found none. Then I gave up and took the lower unit to an authorized Honda shop to find leaks and make repairs. The shop called and said they pressure tested and found no leaks. They added that they went ahead and put new seals on the oil drian and fill holes, anyway. And, they were going to replace the prop seals, but the prop hub is corroded really bad, and they were afraid taking it apart, being as corroded as it is, it may not seal back up properly, the way it originally was. They said that it's not leaking right now. He added it's holding the pressure, so it is good to go operational.

Does anybody have thoughts on what, if anything, to do next, outside of waiting until next fall and seeing if the oil comes out milky again?

Thanks,

Mark010
 
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