Re: 40hp merc
Off the top of my head, likely areas where oil (or oil/gas mixture) or oily gunk can get into the cooling water:
1) Oil seals at bottom of crankcase--whether oil seals are bad or crankshaft sealing surface is worn/scored at the seals--along with bad exhaust adaptor plate (plate between the powerhead and the mid-unit or "leg") gasket, which would allow raw oil (or oil/gas mixture) to mix into the cooling water...and conversely, cooling water into the crankcase
2) Same bad exhaust adaptor plate gasket, but at the exhaust area, which would allow burnt exhaust to mix into the cooling water (and vice versa)
3) Bad gaskets or corroded aluminum in the exhaust cover and/or exhaust baffle area, which would allow exhaust gunk--here, could be burnt or unburnt--into the cooling water...as well as water into the cylinder through the exhaust ports
IF it was 2) or (especially) 3), you'd likely also be getting some nasty foul goo out the thru-prop exhaust.
And IF it was 3) (which is much more common in saltwater motors, BTW), you'd be for sure getting water into the cylinders and would notice a definite loss of power, as well as some evidence of wet, nasty gunk on the plugs--but in this case I would not expect it to travel to the vents where you are seeing it...so maybe we can rule this one out.
So based on the evidence so far, I'd go with it likely being option 1), which is unfortunately very common on these aged motors, salt water or fresh. If that's the case and it's not dealt with, the crankshaft, crank bearings, rod bearings, etc. are eventually going to decide they do not like being lubed with water.
So I'm curious to hear how the plugs look. And also, this oil coming out the vents, does it seem like "fresh" unburnt oil/gas, or more like nasty burnt gunk?
