Yamaha 40hp water in gear oil, but pressure tests fine.

[tvpierce]

My motor is an '89 Yamaha 40hp 2 stroke, 3 cylinder. I just bought it last fall, so am going through it this spring.

First off, I never leave the boat in the water. I drop it in, use it, then pull it out -- and I'm on fresh water.

The first time I had it out, I came back, checked the lower unit oil, and it was milky. So I drained it, replaced the washers on the fill & level holes, and refilled it. Took it out a couple weeks later: came back, checked the gear oil, and it was milky again. So I drained it and refilled it. Took it out a week later. Checked the oil when I got back, and milky again.

So I order new seals.

Pull the lower unit, the water pump and the seal housing. There was a little oil on top of the seals, so I figure that's where my problem is, but I decide I want to pressure test it just be be sure. I pressurize it to at 8 psi. Holds pressure fine. In fact, I let it sit pressurized overnight (8 hours) and it was still at 6.5 psi in the morning! I figure that's a pretty tight gearbox. Should I just replace the seals anyway?

Thanks in advance for your help!

 

[JB]

Re: Yamaha 40hp water in gear oil, but pressure tests fine.

Did you use new gaskets under the Vent and Drain/Fill plugs? A pressure test is not foolproof. Water infiltration happens when the internal pressure is below ambient, not above. To do a pressure test you have to remove at least one gasket. It is possible that a seal or gasket admits water when a hot gearcase cools and internal pressure goes below ambient.

I would cost near nothing to replace those gaskets. Worth a try. Good luck.

 

[tvpierce]

Re: Yamaha 40hp water in gear oil, but pressure tests fine.

Yes, new gaskets/washers on the drain/fill and vent plugs. I hadn't even thought of how thermal expansion/contaction would impact things.

What's interesting though is that the oil and water are always mixed well -- uniform milky color. Which I thought was an indication it's taking on the water while the gears are moving. If it was taking on the water only during cool-down -- with the motor stopped, wouldn't the water tend to be separated from the oil?

In any event, good to know that the pressure test isn't fool proof. I already have the seals, so will go ahead and replace them.

Thanks!

 

[Capt Ken]

Re: Yamaha 40hp water in gear oil, but pressure tests fine.

Pressure test is for checking for oil leakage. Vacuum check is to check for incoming water leaks.

 

[tvpierce]

Re: Yamaha 40hp water in gear oil, but pressure tests fine.

I'll swap out the upper seals then, and do a vacuum check once I have it buttoned back up.

Thanks.