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2005 Cobalt 263 496 Mag Bravo 3, bought at the end of February with 512 hrs from a Cobalt dealer in Grand Rapids, MI...seller bought a new boat and sold this through the shop on consignment. I didn't receive any maintenance records from the previous owner, however the boat had been serviced at this dealership since at least 2018 when the service department changed their computer system and didn't transfer older records. Records for 2018 to February this year showed typical recommended annual maintenance services were performed. The gear lube reservoir was at the "fill to" mark when I left the dealership. Drove it 2700 miles (three 12 hour days in rain and snow) home to California--note that the outdrive was in "full up" position but I did not have trim cylinder trailering clips for the trip home. When I got it parked at home I lowered the drive, to "protect" the transom bellows, and about two tablespoons of gear lube mixed with water slowly leaked from the bottom water intake hole of the outdrive lower. After placing a drain pan under the drive, I opened the engine hatch and saw that the reservoir had approximately 1/2" of lube remaining in the reservoir. There was a very limited amount of "blue lube" spots in the bilge in the area below the front of the engine but absolutely not the amount missing from the reservoir. I checked the cap on the reservoir, it was tight (actually over tight, and had to use channel locks to get the cap off) and with the cap gasket in place. I checked the entire underside of the engine, reservoir, and gear lube hose routing to the transom (as best I could) and found no trace of leaked lube. I'm assuming that it all leaked from the outdrive but there was no back-spray on the transom or trailer frame, and the outdrive didn't have a "film" indicating an active leak. I'm puzzled by the disappearance...possible that it leaked out early in the trip and was "washed" clean by continued travel in rain/slush but in any event, it is what it is. I refilled the reservoir at that time and have not seen any new evidence of leakage UNTIL today when I noticed the lube drop in the photo--there is no sign of additional drops on the concrete, and the reservoir does not appear any lower.

Side note, I've yet to operate the boat other than to run the engine in the driveway--not in gear--to troubleshoot a starting issue (bad neutral start safety switch) and check for any other obvious problems (over heating, idling, etc) before our first sea trial scheduled for July 10th. I know, late start but remodeling our house has been in the way...I feel lucky to have been able to swing a remodel and a boat!

Anyone have similar experience...thoughts on any suspect cause(s) such as specific seal(s) in the outdrive?

Forget the monitor (gear lube reservoir). First thing I do is buy new out-drive plugs and vents and about a dozen little o-rings and replace the plug and vent with new o-rings. Also buy a stern drive hand pump to fill the outdrive bottom up.

This assumes there are no leaks in the lines above. How many hours on the 2005 engine? Age and hours, or just age with things like bellows, mean rebuild and/or replacement

Thanks for the prompt reply and sorry for such a long post, I know it is a bit too long...I read too many that do not have all relative info and see too many threads with more questions than answers. Was trying not to be that guy. First sentence...512 hours and, unfortunately, I don't have any records so cannot say what services have been done on the outdrive.

Have new outdrive plugs (came with o-rings) and manual gear lube pump...will drain and refill before the first outing. Not familiar with the "vents" you referenced, and their locations...will review my #39 Service Manuals for the stern drive. The bellows appear to be in serviceable condition...no age-related cracks in the rubber when in the up/down positions. Depending on the source of the leak (seals within the transom assy, bell housing, upper/lower mating, etc) will replace the bellows if disassembly is required...otherwise, likely over the coming off-season.

Still looking for potential