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Author Topic:   Water in lower unit
whaler1234 posted 07-07-2006 03:21 AM ET (US)   Profile for whaler1234   Send Email to whaler1234  
I changed the lower unit oil at 30hrs in my '04 90hp Mercury and found water in it(milkshake color).

After putting another 10 hours on the motor, I decided to change the lower unit oil again to see how it faired this time. To my surprise, the oil came out clean.

Can anyone explain this phenomenon?

The first time I changed the oil, the mixture settled to about a 70/30 percent ratio of dark green to light green.


bsmotril posted 07-07-2006 09:41 AM ET (US)     Profile for bsmotril  Send Email to bsmotril     
Perhaps the seal on the drain or vent plugs was not good and leaked water into the unit. When you changed the fluids, you removed and replaced those plugs, correcting the problem. BillS
Chuck Tribolet posted 07-07-2006 10:08 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Your merc has a place to directly hook up a hose for flushing
(no muffs), right? You may have flushed with a higher than
usual pressure.

My 90 Evinrude has one, and the manual specs 50 PSI max.
turned out one place I flushed had 125 PSI static pressure
and that had pushed cooling water past the seals. I rigged
a pressure regulator from some drip irrgation parts.


sosmerc posted 07-07-2006 10:53 AM ET (US)     Profile for sosmerc  Send Email to sosmerc     
I highly recommend you change the little seal washers every time you inspect or replace your gear oil. I use a mechanical impact hammer to gently tap the screws tight when I am done (NOT AN AIR IMPACT). I recommend Merc's high performance gear lube...I use it in ALL my customers outboards.
The shift shaft bushing/seal is the weak area of that lower unit as far as water getting in. After a long run the lower unit can get pretty warm inside. Then, if the unit sits down in the water, it can draw water in due to a vacuum being created in the gearcase from the pressure and temperature change. Water can get in through the driveshaft and propshaft seals the same way, if they are worn...but usually it's the shift shaft seal.
To properly inspect, the unit should be left vertical and not run for several hours. If you open up the lower screw and get STRAIGHT have a problem that needs attention. But I do not get overly concerned if the gearoil is just a bit milky...just change it. On my high performance boat, I inspect/change the gearoil before every outing. But for most folks, I would recommend an inspection and possible change several times a season to be safe. It would also be wise at this time to pull the prop and look for fishline behind the rear thrust washer.
Changing gearoil is cheap compared to replacing seals and bearings.

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