Filling lower unit

[wilbur1]

I was changing the gear oil on a 2015 115atlss. On the last pump the container ran out of fluid and pumped in some air. My question is if I leave the top plug out a few days will this air escape ? I put the top plug in loosely and raised and lowered the engine a few times then set back to level and removed the top plug for a day.Should I drain and start over or is it OK. I would think with the plug out any air would work its way up and out. Suggestions Thanks Bill

[Hoosmatroos]

Yes, air would work its way out. You can also plug the top hole and raise and lower the engine. The oil is pretty thick so leave it for a while and fill up as necessary.

[wilbur1]

Thank you Sir. I did this Sun. morn and I did raise and lower a few times. I will give it a few more days. So far the level is at bottom of top hole. Thank You.

[Harper]

When you fill the lower unit, no matter which hole you're injecting the fluid into, it goes into a large reservoir. So if you're doing it by the book and pumping in from the lowest hole, the drain hole, any air injected will immediately rise to the surface of the fluid inside that reservoir, just as fast as the bubble can rise in the thick fluid. It's not like you're injecting into a tube where it has to work its way up over time. Your bubbles of air will rise to the surface of the fluid within a minute or two, max. And there is no need to tilt the motor to facilitate the movement. I would not recommend leaving the top plug loose or out at all. To do so just invites you to forget to tighten it later (at least if you're old like me!)

If you want to know how long it takes for a bubble of air to rise to the surface of a reservoir of cold gear lube, just pour a quart of gear fluid into a quart mason jar and with a plastic tube, blow a few bubbles of air into the bottom of the jar. You'll see that it takes very little time and no rocking of the jar for the bubble to rise to the surface.

[Hoosmatroos]

Mike, agree but (there is always a but...) between the gears it is possible that an air pocket is forming and not all can rise to the surface. Specially when the motor is not completely tilted down.

[TxAg66]

So, given that air will rise (even in thick, viscous fluids), why not *fill* from the top hole? (And, yes, I realize the book says to fill from the bottom/drain hole.) Not trying to be a smart-aleck here .. I do it by the book but, every time I do it, I can't help but wonder if it *really* needs to be done that way.

[Harper]

Mike, agree but (there is always a but...) between the gears it is possible that an air pocket is forming and not all can rise to the surface. Specially when the motor is not completely tilted down.

Martin, my whole point, really, is that we're obsessing over nothing here. I won't argue whether there can be a few bubbles just sitting somewhere in the gear set or under it, or in some pocket caused by flashing from the casting of the gear case itself. But I will argue that that bubble, or those bubbles will surface at some point, and when it does/they do, it will not change the level of the fluid but a micron or two, nothing you could ever perceive by any means of measurement. I do object to the idea of leaving any plugs loose. That accomplishes nothing. It's not as if air would have to escape the fluid reservoir in order for a bubble to surface. The total volume of air and fluid would not change as a bubble works its way to the surface of the fluid. The fluid is not effervescent. No pressure needs to be released.

I end up injecting some air into the mix every time I drain and fill. I fill with the motor trimmed level. When I reach the proper level of fluid, I button it up, and I'm done. No tilting of the motor up and down, no loose plugs. Done.

So, given that air will rise (even in thick, viscous fluids), why not *fill* from the top hole? (And, yes, I realize the book says to fill from the bottom/drain hole.) Not trying to be a smart-aleck here .. I do it by the book but, every time I do it, I can't help but wonder if it *really* needs to be done that way.

John, it can be filled from any of the holes. It's just easier to see when you reach that full point if you fill from the bottom hole. Fill it from the level indicator hole and you have to keep removing the fill tube in order to check the level. I don't know about the smaller engines, but my DF150 has three holes. The very top is a vent hole (I don't ever open it to fill, since air will vent out the level hole anyway) then there's the level hole, and of course the drain hole, which I also use to fill. If the smaller outboards have only two holes, then you'd have to fill from the bottom hole in order to provide a means for air to escape during filling.

If I filled it from the level hole, I'd have to open the vent hole above it to let air escape during the filling process. And would still have to remove the tube to check the level. I've never tried it, but if I filled from the very top hole, the vent hole, it seems that some of the fluid would dribble out the level hole situated right below the vent hole, since the level hole does double duty at that point as it also vents during the fill.

Got it?

[TxAg66]

Mike, thanks. 👍

[Hoosmatroos]

Martin, my whole point, really, is that we're obsessing over nothing here. I won't argue whether there can be a few bubbles just sitting somewhere in the gear set or under it, or in some pocket caused by flashing from the casting of the gear case itself. But I will argue that that bubble, or those bubbles will surface at some point, and when it does/they do, it will not change the level of the fluid but a micron or two, nothing you could ever perceive by any means of measurement. I do object to the idea of leaving any plugs loose. That accomplishes nothing. It's not as if air would have to escape the fluid reservoir in order for a bubble to surface. The total volume of air and fluid would not change as a bubble works its way to the surface of the fluid. The fluid is not effervescent. No pressure needs to be released.

I end up injecting some air into the mix every time I drain and fill. I fill with the motor trimmed level. When I reach the proper level of fluid, I button it up, and I'm done. No tilting of the motor up and down, no loose plugs. Done.

Agreed

On the small motors (up to 15hp) it is possible to have a significant amount of air trapped. On the bigger motors I totally agree with you.

[wilbur1]

Thanks Guys. Its all buttoned up waitin on warm weather. Thanks