You might want to think twice...or three times about this purchase. Or, about getting too deep into one of these engines. While I LOVE the 8V71, they can be a fairly steep "learning curve" experience for even an accomplished mechanic. Add to that they are in a boat so everything is pretty "tight" as far as room goes and that the components, like cylinder heads, are really heavy....well, it's not for the faint of heart.
So, lets talk about the issue: Water in the oil. "How could that happen?" you might ask.
If this is a closed cooling system with a radiator cap (you mentioned anti freeze so I'm assuming here) the maximum pressure that you are likely to see in that system is around 10 psi or less. Usually less on a boat. So, let's skip to the oil system...where you can see upper pressures of 50 to 60 psi with an average of about 40.
With those numbers, it is plain to see that, if there is a leak where there is an "interface" seal between the two systems, it is MUCH more likely that the higher pressure oil would infiltrate the lower pressure coolant and, thus, you would be faced with an "oil in the water" complaint. Not always but mostly.
What that pretty much rules out would be the oil cooler has a leak. That might be a repair that you could tackle and win fairly easily but it is just not likely that is the case here. Another example of a "simple explanation with simple repair"
Would be the water pump seal leaking. Changing a water pump on one of these is "no day at the beach" but is perfectly doable for a determined "novice"
But, what is much more likely to be the problem would probably fall under one of two headings:
Cylinder head water grommets...or
Cylinder liner seals.
Both of these scenarios will become a fairly large undertaking for you with cylinder head removal only at best and lower end engine work....remove rods and pistons....in the event of liner seals gone south.
The removal of either head will involve a complete tune up assessment and re-set. Just learning to do a "decent" tune on one of these engines can take a guy a year or two to get down pat. I did know some "phenoms" in my time that could get the manual and knock it out in about three hours the first go round but these dudes are like "idiot savants" that can sit down at the piano and play a concerto the VERY FIRST TIME! In other words...It ain't happening!
If you are SERIOUS about buying this boat and the seller is SERIOUS about selling it to you then my advise, for your own preservation, is to either find a GOOD, professional, Detroit mechanic WITH REFERENCES....that you ACTUALLY CHECK UP ON...to do an evaluation of these power plants. IT WILL NOT BE CHEAP! BUT, it could save you THOUSANDS in the long run or keep you from having a boat that you "Cockpit cocktail at the dock" in but never take out. If the seller balks at doing this in any way...RUN!
Barring that, get yourself a service manual (not really all that cheap either) and read up on what it takes to do some of the procedures I've listed. You could pressurize the system, take the inspection covers off and at least have a look at the liner seals. Just doing this preliminary inspection will give you a teeny taste of squirming around in tight quarters in the bowels of an engine room with a large displacement V engine. Most "newbies" call it quits after the first shower.
Also note that ANYTHING that pops up on the engine in question will be happening to it's mate in a very short time. So, when you plan a repair, plan on doing it twice.
You will find this engine to NOT be DIY friendly with "special tools galore" required. But, if you get by all that, you will discover a BULLET PROOF, get you there and back, BAD A$$ diesel engine in the 8V71!
