2005 5.7 Mercruiser water in oil

I have a 2005 5.7 mercruiser mag mpi fresh water cooled w/dry joint exhaust. Brought the boat home last fall and tried to winterize port engine, it was water locked, pulled plugs cleaned with wd40 started it an then proceded to winterize the raw water side. My mechanic said it "dieseled" and not to worry. Fast forward to spring oil change, it water locked again only this time found liquid in one cylinder. Once again cleaned w/wd40. Before I started it there was no trace of water on the dipstick. Started it and let it run about 10 min. w/water hose hooked up, shut it off went to change oil and it was very milky w/ an extra quart in just 10min. of running. Since the exhaust manifolds are fresh water cooled this leaves the risors as the first place to look correct? the inside of the exhaust manifold port side,had some surface rust but no liquid(only 5 days since it had been running). Compression test showed it had around 165psi in each cylinder. please tell me you think it is the risors or the heat exchanger? maybe? I don't understand how a freshwater engine full of coolant could crack the block, worst case senario. Thanks for your time.

Re: 2005 5.7 Mercruiser water in oil

You aren't going to get water in the oil through the exhaust with the engine running.
Did you check the antifreeze to see how low a temp it's good for? Did the level of antifreeze drop the same amount as what was in the oil?

Re: 2005 5.7 Mercruiser water in oil

The coolant level never dropped at all. The coolant is the standard prestone GM orange. I live in Maryland the winters aren't that bad. Thanks for the quick answer

Re: 2005 5.7 Mercruiser water in oil

No change in coolant level. The engine would still start, just oil became milky. Pulled the risers off and there was no water marks around the gasket area that would indicate a poor seal or leak. Had the risers pressure tested and no issues.

Found the cause. Mercury/Mercruiser Service bulletin No. 2001-13 Water Intrusion
Water coming back in through the exhaust system. One of 4 reasons: 1) Engine running conditions 2) Boats waterline 3) Water Ingestion 4) Engine Exhaust System component.

Our is 3) Water Ingestion
Doing throttle chops can cause water exiting from the exhaust elbow to be drawn backward into the engine...

There is a lot of water sitting in the muffler. In the process of warming up the motors, we had reved it up a bit. Since becoming aware of this situation we no longer rev it up and the oil remains clean.