Water in Outboard Gear Oil, Maybe

Hi Everyone,


I have a 2004 2-stroke Mercury outboard motor. Last week I was doing some regular maintenance on it, and the last thing I did was inspect the oil in the lower unit.


The last time the oil was changed was May 2020, and I didn't do it myself, so I didn't see what type or color of oil was put in to the lower unit. Usually I use Mercury Premium SAE90 Gear Lube Oil, which is blue.


I decided to open the top screw of the lower unit first, to see if I could tell the amount of oil in it, and thought I would just take a small amount of oil out using a straw, or something like that, to inspect its condition. I know from previous inspections that I am not very quick at putting the lower screw in and often lose more oil than I would like when I do this.


What I found was that when I undid the top screw, oil started to come out of the hole. I probably should mention that the engine was mounted vertically at the time. I collected some of the oil, and you can see its color and condition in the attached picture.


So my question to everyone is, since I have never had water in my gear oil before, is that what I am looking at? Or is this just a different color of oil?


Thanks.

It appears milky enough that yes, it has water in it, unless you ran it right before checking.

One way to check it is to (carefully) heat some of it to see if it starts splattering which is the water boiling out of the oil. I was shown to do this by dipping something metal like a screw driver in the oil then lifting it out and heating it with a flame. But just looking at the picture...

I'm 99% sure that you have water in your lower unit. You need to pull that unit, expose the water pump housing. Check to see if there is any of that same colored oil coming from the prop exhaust. If so the gear assembly needs to be resealed with new gaskets and shims. The most prevalent area of water leaks is the O rings on the bottom of the water pump assembly where the shaft enters the lower unit. It is a DIY project. Many videos on U tube to help you out.

You have water in the oil. I've dealt with this several times. A seal has worn enough to allow water to enter the gearcase.

I would let it drain overnight into a pan. Then fill the gearcase with alcohol which will absorb the water, let it sit for 10 minutes and drain that also. Leave it open to drain any left in the gearcase.

Get it to the shop unless you feel up to the task.

In my case, an 8HP Yamaha, the leak was the shifter rod boot. Looks like an accordion bellows that enters the gearcase and seals the shift rod entry from water entry. It was split. If you have something similar check it carefully or replace it.

Thanks everyone for your replies. I was wondering what I should do to 'dry' the lower unit before putting new oil in. I have replacement seals and the necessary tools to do this repair myself, so that's good.


Now one last set of questions.
Why would it matter if I had just run the outboard?

Run the O/B with the watered oil?

You would have wrecked the gears and bearings in the gear case.

Does the fact that the oil came out of the top screw hole indicate that there is water in the lower unit? In effect, the gear case became overfilled with the addition of water?


Is there any other way to clear out the water/oil mixture?


Any suggestions how I can inspect the gears?


Thanks again to Everyone for their answers.

I am not an O/B mechanic so have only my own machine to work on.

The TOP screw hole meaning the oil fill hole?

Usually the gear case is well below the water line outside so if the leak went on long enough maybe it could fill untill it overflowed. The leak source could have been higher up as mine was.

I paid little attention to the actual level of fluid. I simply opened the drain as I do each year and white oil exited.

I took a chance and simply redid the water pump impeller, the shift rod bellows and associated seals for both. That stopped the water entry. That was 4 yrs ago and never has there been water since.
My leak was not the gear case seals.

THe mechanic I originally handed the engine to , when I pointed out the split bellows said no, that is not the cause and left it in place.

When I had to do it I could see the bellows could easily leak water directly into the gearcase. He was wrong. Anyhow enough of that. Actually I trusted him and the same thing happened the next year so I did it myself over the winter.

Be sure that nothing else allowed water in such as leaking seal higher up like my shift rod bellows.

On mine once the gear case was off I could see the gears quite well. No sign of any corrosion. THe oil is not waterproof but highly water resistant so even though it was compromised it still protected my gearcase.

Do the alcohol flush and that should clean any remaining water based on my experience.

Yup have had the same thing a few times over the years. Yes the fact that fluid came out when you took out the upper vent screw means that water entered overfilling the gearcase. I usually replace the propeller shaft seal ( often find fishline wrapped) then buy some cheap lightweight box store oil and fill gearcase from bottom and let it run out the top vent for a little while, where the water is lighter than oil it usually flushes out. Drain the light oil and then refill with gear oil and recheck after a few hours use. Please recycle the used oil

While I'm at it I usually drop the lower unit and if it's been a while, or the engine sat dry for a while I change the water pump impeller and seals. Remember to prelube the impeller when you install it.

Thanks again everybody for your replies.


I like the idea of using oil to flush out the old oil/water mixture, as I have more oil on hand then alcohol. I am expecting to find some fishing line has fouled the seal because I was in an area with lots of fishing traps and there was a lot of stuff floating in the water.


I watched a reliable mechanic replace the water pump 2 years ago, and although I know it is recommended to replace it every 2-3 years, I also know that it is a really big job to get at the pump in a Mercury outboard. I have a really strong telltale stream of water when I run the engine, so it seems that the impeller is in fine condition right now.



I will be able to do this work next week, and will post a picture of what I find then.


Have a great weekend!

There is only two places water can get in.

(1) the prop shaft carrier has an O ring, and shaft seal. This requires that you pull the carrier assembly to replace the O ring and shaft seal.

(2) The water pump base and seal. If it is more than a year old buy a complete water pump kit. It comes with a new base, upper housing, and impeller. Be sure to lube the impeller and seals before installation.

Former Mercury Marine 15 Year Master Mechanic.


PS Remember that water is heavier than oil.
If you pull the bottom plug first and water runs out you've lost your seals!

Hi Everyone,


For those who offered me help and suggestions, a quick update on my lower unit oil problem.


When I started the process of draining the oil, the first thing I did was remove the upper vent screw, and lo and behold, there was NO gasket sealing it to the unit. That will teach me to let someone else I don't know work on my outboard. And to let someone work on it while I am not watching.



I drained the water/oil mixture and replaced it with new oil. I have just started using it again, and will replace this oil with fresh in a few hours. Hopefully I will find that this has cleared out the unit and that the source of the leak was the missing screw gasket.

Pelagic said:

Hi Everyone,


For those who offered me help and suggestions, a quick update on my lower unit oil problem.


When I started the process of draining the oil, the first thing I did was remove the upper vent screw, and lo and behold, there was NO gasket sealing it to the unit. That will teach me to let someone else I don't know work on my outboard. And to let someone work on it while I am not watching.



I drained the water/oil mixture and replaced it with new oil. I have just started using it again, and will replace this oil with fresh in a few hours. Hopefully I will find that this has cleared out the unit and that the source of the leak was the missing screw gasket.

Click to expand...

That would do it.

Hope it was that and good you found it.

Just to be sure pull the prop and check for fishing line. You'd never know you wrapped some but it will wreck the shaft seal. BTDT.

jg winks beat me to check for fishing line cutting the seal. Mono line can cut into a seal over time, remove prop and check the prop shaft for line....

I would replace the top o ring, drain well, flush with a bit of diesel or WD40, drain well and refill (make sure its warm from room temp or warm a bit with a hair dryer). Run for a couple hours and drain again. If good, refill and continue operating.

If bad, start on all the other seals mentioned.