This one's kinda strange. A friend has a 225 Yamaha VMAX that, after several years and way past warranty, has started to lose significant lower unit oil. Confused a bit, he's starting to check it every month or so, and top it back off.

The confusing part is that it's not showing obvious oil on a surface to pinpoint a leak, plus it's not showing water mixed in the oil left inside each time.

He's getting a seal kit and going to replace all the seals top and bottom, hopefully hitting a bad/weak one, figuring it's a side of a seal that may be letting oil out while the other side is good enough to somehow keep the water from coming in, yet maybe let air back in later. See why it's strange and confusing.

If you've solved something like this before, share a comment, I'll pass it on. Thanks, guys. -ray

not familiar with that particular motor, but here's a possible scenario...most lower units have drain hole at the bottom and a vent hole at the top...if you fill it from the vent with the drain plugged, you get air pockets. maybe he didn't fill it completely on his last gear oil change?

The drive shaft seal, meaning the inner most, since there is two back to back. One is to keep the oil in the other is to keep the water out. When you run the engine, the water pump impeller is producing a vaccumn just above the drive shaft seals and is litterly sucking the oil out. That is why you don't see any visual signs of oil leaking. Not uncommon to see this but weird to say the least.

jtexas - good suggestion, he is topping it back off filling from the bottom to push air up/out the top vent and the subsequent loss is recurring after a weekend of use, so oil is going somewhere as opposed to not there to start off. -ray

Mike - he's hoping something like that may be what is happening but replacing all the seals just in case. Not seeing any residue from weep holes or the pee hole on the upper motor.

I have a 225 Mercury EFI not a Yamaha. During an annual maintenance not long into the life of my motor, I had to have a stock water pump replaced with a heavier duty one because the thinner plate on the bottom of the pump bulged up enough to score my impeller.

I think that was from lower unit pressure on the base of the pump. I obviously don't understand the pressures and vacuums in play around lower units. My friend explained that some inner seals keep the oil in while other outer seals keep the water out similar to the concept behind the wheel bearing grease seal on my bearing-buddy trailer hub (except that one is a single dual lipped seal).

Thanks for your insight. I'll pass it along. -ray

You may know but when replacing the seals it is real important to sand the seal surfaces on the shafts so the will seat correctly. Good Luck

Ray, you may want to have a Merc dealer check and see if your Lower Unit needs an updated driveshaft seal carrier(one that bolts down). I have seen what you stated in your article happen many times.

Thanks Gary, I'll pass along sanding the seal surfaces on his Yamaha shafts. My friend's an auto mechanic and may already have been coached to do that.

Thanks Alan, my Merc's ok, my friend's Yamaha has the disappearing oil problem. I incorrectly referred to my Mercury water pump as a heavy duty replacement, when in fact I believe now it was probably the high performance seal carrier plate (that bolts down) that was replaced on my lower unit.

Alan, you can see I'm still being educated on terminology. When I was talking to my Mercury mechanic today, he quickly pointed out that my pump was the same part model as before, he simply updated to a bolt-down plate to rectify my old "bulge" problem a few years back. That's likely the carrier you mentioned.

Sorry I mixed Mercury into this Yamaha thread, although the seal problem could cross brands. Having discovered the cost of a complete seal kit, my friend is now thinking obout targeting just the mentioned drive shaft seals on his Yamaha first.

Plus, he recalls there may have been a slight hiss sound as he removed drain/fill plugs when checking his Yamaha, and that would add credence to Mike's theory about what the pump and seals are doing.

Thanks all, I'll pass along what you said. You've likely solved this stange one. -ray