Has anyone seen this one before ?
Low hour 502Mag/B3 laid up for 3 years due to pandemic and job mayhem. Got the mechanic on the case to go over and re-commission everything and the first thing we noticed when the engine hatch was opened was that the outdrive reservoir bottle was empty . It was at the proper level at the time of layup.
A week prior to layup it had an early case of Bravoitis tended to and in the process the outdrive oil transfer nipple had to be replaced so we know that isn't broken.
Pulling the top vent screw on the outdrive revealed that the oil level was an inch or so below the screw.
There was no oil in the bilge and no oil showing anywhere on the lower unit. Assumption ? Top universal joint oil seal leaking.
Take outdrive off and the U Joint bellows is clean and dry !
Pressure test outdrive and it pressurizes and holds fine . Vacuum test it fails but not wildly. Turn unit upside down and pressurize again and no oil leaking from top u/joint seal.
Mechanic now has outdrive at his shop but it looks like another boatless season at this stage .
Has anyone seen anything like this before ?
Technically , failing the vacuum test but passing the pressure test should say that the unit can't leak oil .
I'm thinking the aliens came looking for oil one night and just transported a quart or two neatly away .

Air trapped during installation of fluid. Had it happen several times. I always re check mine Every outing x4 after fluid change. and I have seen them drop enough to empty the reservior bottle.

Chris

Yep. Especially if you only fill the drive to the fill screw and then directly pour the drive lube into the reservoir.

And don’t bravo drives not hold a vacuum?

^ Good common sense reply but I've been tending to this unit for 20 years and after a fresh change / re-fill it settles down to the right level after a week or so . It had completely stabilized 3 months before layup.
And it's always filled from the bottom plug with the top plug in until the oil is at the correct level in the reservoir bottle .
Generally speaking the bottle will only go down an inch while it settles and then need a small top up.

B3's are notorious for developing a leak, more so with non-use. Had a brand new one leak and I mean new from Merc
After a bit of use it sealed right back up. Several years later and still ok

That's kind of the line we're thinking about but we need to figure out how it was able to make such a significant amount of oil vanish without any clues .
Our mechanic is no slouch and has been top Merc certified for decades and he almost immediately ruled out the 'bubble in the unit' theory.

Yeah , but have you seen them drop to below the top vent screw (installed while filling) when the unit was filled from the bottom to the fill line of the reservoir bottle and the amount of oil that went in was to spec ?

Mine never lost that much, you might need a reseal
It sat for 3 years, so unless it was sitting on gravel it should show somewhere. I'm assuming the drive has been pulled so its not all in the bellows?

First what are you calling to spec? Second oil does not disappear . It went somewhere like an Air void. You stated you filled it from the bottom plug with the top plug in. Should be filled with the top plug out till it makes a mess then the top plug reinstalled then filler her up by continuing to pump. Just a small amount of sludge that did not let all oil out can create an air pocket with old oil above it.. Making you think it is full. Try draining the oil/fluid out, put the plugs back in and cap on and let it sit in the sun for a couple hours. Then pull the plugs back out. you might be surprised how much will then drain out that was trapped. Just Ideas for you. I had one I thought was perfect for a month. Hit a big wave and then I felt a small vibration. Was 100 yards from island so just went at idle to the island. Checked fluid bottle and it was very low but alarm was not going off. Filled bottle up and left a couple hours later. Idle for 5 min then took off and no more vibration. At the dock I did notice bottle had dropped slightly. I was still nervous about it so I drained and refilled with procedure above. 6 months now and no problems. I also now make sure drive is level and trim up and down a couple times to make sure all oil is out. I obviously had enough air to empty the bottle. Mine takes almost 2 quarts. Dont even have enough left in the second quart to feel comfortable leaving the dock..

Chris

Yes , that's the thing , there's no sign of the oil anywhere . Bellows , bilge or on the ground .