Mercury 3.5 Hp Water in oil

[Nichole340]

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.

 

[SailingLoto]

You will need to pressure check the lower unit. It's leaking somewhere... You just gotta find it.

 

[Rich Stidger]

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.

 

[Nichole340]

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.

 

[Rich Stidger]

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.....

 

[sailcapt340]

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.

 

[Nichole340]

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.

 

[Crazy Dave Condon]

Your Mercury 3.5 was built by Tohatsu who also builds the Nissan 3.5. It helps to know when looking for parts

 

[Nichole340]

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.

 

[sailcapt340]

I know this is an older posting, and don’t know if you have done your repairs to your lower unit. I just dropped the lower gear unit from my 3.3 Mercury to replace the impeller. Admittedly, it’s been a while since I’ve replaced it.
I found oil seeping from the under the Pump Housing.
I’m bringing this up because there is an oil seal inside the housing. And if oil is coming out, water could be going in and contaminating the oil in the gears. You said you found air from your pressure test escaping at the O Ring Seal and I hope it is your leak area but it’s worth looking at the Pump Housing of your outboard.
I also found a lot of salt deposits in the Housing which was slowing the flow of water throughout this system. I have enclosed some photos for you to look at. The salt you see is what’s left after I tipped it upside down and banged out the big chunks.
Next will be to remove the Pump Housing and replace the Oil Seal. Not looking forward to that, as that Housing is in there pretty tight on a good day. And not a big fan of tapping on 20 year old parts to loosen them. But that gasket needs to be replaced at least.
So I’m not wishing you more work as much as I am hoping you don’t see what is in my pictures for yourself.

 

[jviss]

I have looked into seal replacement for the lower unit on my Mercury two stroke 15HP. It's not trivial! It requires several seal parts, and several special tools.

I have been collecting lower units over the years on craigslist and eBay; I think I have four of them now.

No one will fix one for anywhere near the price of a new unit, which was about $1,500 last time I checked.

The correct way to check the seals, in my opinion, is not to pressurize it, but ti pull a vacuum, and see if it will hold the vacuum for a day. You can do this through a modified oil plug, and a handheld vacuum pump. This is how underwater camera housings are tested, too.