Mercury 3.5 Hp Water in oil

Jan 12, 2019
59
Hunter 340 Narragansett
A while back, I had a problem with getting oil into the lower unit of my 2000 3.5 HP Mercury Outboard. That was solved by a simple clean out of gunk in the upper fill screw.
But now I have water in the oil of the lower unit. The oil was milky white when I drained it out just recently. And a very noticeable lack of oil drained out. About 4 ounces, I'm guessing.
I never saw oil in the water around the outboard this season. But I did loose oil.
The only issue I did have was once I noticed no water was coming from the outlet hose connected to the impeller housing. But that was fixed by cleaning out the vent holes in the housing that were clogged with mud. Ran perfect after that.
As far as water in the oil, all screws (4) are tight holding the lower unit and prop seal on.
Anyone else experience this and how did you fix it?
Thank you for the help.
 

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Feb 10, 2004
3,520
Hunter 40.5 Warwick, RI
Water in the lower unit is most likely from the prop shaft seal. Picking up a piece of fishing line can damage the seal and cause a leak. Replacement of that seal is the fix.

There is another seal at the top of the lower unit for the drive shaft. That one could be leaking as well.
Replace those two seals and you should have the leak stopped.
 
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Likes: Nichole340
Jan 12, 2019
59
Hunter 340 Narragansett
Thanks to both of you. I did some research on the parts list for this engine and feel, like you it’s the seals. I also think I need to replace the grommet that holds the water pickup tube from the impeller pump housing. It’s just to dry and almost brittle.
Probably not the issue, but water being water, it can find the most least likely entry points.
I also want to try the pressure testing of the unit. Basically, just because I want to try that and learn that process.
Thanks to both of you for your kind responses and help.
I‘ll keep you updated.
 
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Likes: Rich Stidger
Feb 10, 2004
3,520
Hunter 40.5 Warwick, RI
I also want to try the pressure testing of the unit. Basically, just because I want to try that and learn that process.
I'd like to know how you pressure test a lower end. I'm guessing that you somehow connect an air compressor to one of the grease ports and spray the seals with soapy solution and watch. Easier to say than to do.....
 
Sep 26, 2008
341
Hunter 340 Wickford, RI
I've pressure tested my 3.3 Hp outboard by using the screw in fitting from an oil refill pump kit. I screw the fitting into the bottom oil screw, upper screw vent left in tight. I have a barbed fitting in the hose from removed from the pump handle. A hose the fits over the end of my small electric air pump. I hold in on with a hose clamp. I apply a small amount of air into the system, about 5 pounds. Put the unit in a 5 gallon bucket of water. Any bubbles will show.
My best guess is the lower oil seal. Wears out from torque.
Also, not saying you are doing this, but, some use "automotive" 80-90 weight oil in place of marine lower unit oil. There is a difference. Marine oil has water resistance.
Just my thoughts about the issue. Not much can go wrong in there given quality oil, yearly drain and fills and no adverse grounding of the prop in mud.
 
Jan 12, 2019
59
Hunter 340 Narragansett
I’m going to do the pressure test today. And replace the seals, O Ring in the unit. As for the oil used, it is marine lower unit oil so I’m good with that. Just worn out parts. I keep you informed. Thanks to everyon.
 
Jan 12, 2019
59
Hunter 340 Narragansett
Thanks Dave. Yeah this is definitely entering an abyss in outboards. I’ve known for a while that my 2000 Mercury 3.5 HP is really a 2001, 3.3 HP. Even though it is clearly labeled 3.5 HP.
I did pressure test the lower unit and saw just the smallest of air bubbles coming from under the propeller shaft at the seal that has a rubber O Ring gasket. And did feel a slight wobble of the shaft when I moved it side to side. Kind of like what you can feel in a worn cutlass bearing. There were no score marks I could see or feel on the propeller shaft, so I think I’m good there. A costly part.
So I plan on replacing O Ring, Oil Seal and Ball Bearing seal. This should solve the issues of water in the oil.