Mercury 3.3 HP Outboard ....No Oil From Lower Unit Vent

Jan 12, 2019
99
Hunter 340 Narragansett
I'm in the process of replacing Lower Unit Gaskets, "O" Rings, Grommets etc on my 2000 Mercury 3.3 HP Outboard. Had some signs of water in the oil.
After refilling with oil, there is no oil coming out of the top vent.
My thought is simple blockage. I did remove some grime from the vent hole and tried air, but still no oil will weep out.
Any suggestions as to how to clear the vent?
 
Sep 26, 2008
546
Hunter 340 0 Wickford, RI
Nichole340,
I looked for a schematic of that model Mercury, but could not find any. Even my Mercury Outboard Repair Manual details how to drain and fill but the individual drawings are rather simple.
What I do know, is that the Vent your describing, the Lower Unit Top Vent, is a straight line to under the Water Pump Lower Housing, showing in your photo.
If you remove that piece you will see the line and probably a blockage.
You said you were replacing gaskets, you'll need a new gasket for that Lower housing also.
Could be....A small amount of metal filings, and that's acceptable, especially in a 19 year old motor. Over time they fuse together and caused the blockage. Or simply crusted over gunk....that's what I'd prefer.
You can start by trying to hand twist a 1/16 inch drill bit into the vent, using a Tap and Die T Handle, to loosen the blockage. It may work....if not, a drill on very, very, very low speed.
Your only looking to go about 1 1/4 inches in to the Drive Shaft!
Then, when through, go to a 5/32 bit but NO larger. That will open the vent to the Drive shaft.
Test it with compressed air, a tire inflator would be enough air, once free.
When done.....Remember to flush out the housing and gears with a cleaning solvent, WD40 etc, to remove any contaminants, fill with fresh oil and you should be good to go.
Your doing things right after all these years. It's a quality motor and people look for them.
Hope this helps
 
Feb 10, 2004
3,917
Hunter 40.5 Warwick, RI
Mine takes all of 10 ounces. It fills just fine, just won't vent from the top.

Adding photos to help....
You aren't filling with the leg horizontal are you? They are ment to be filled vertically. Maybe when horizontal there is a lot more volume to fill before the vent oozes? Is there any chance that a gasket is somehow blocking the oil vent? I've had mine apart but it was years ago so I don't remember all the details.
 
Jan 12, 2019
99
Hunter 340 Narragansett
Been busy locating anything I could. It's was clogged right behind the top screw. All white stuff, covered by black oil, but hard as a rock. It took a little doing but a small drill bit did the trick, like sailcapt suggested.
Once through I could get a small pick in and scrape out the clog.
Tomorrow I'll clean it all out and add fresh oil.
Thank you everyone for the help and support. Hope I can return the favor sometime.