Milky engine oil issue after only maybe 4 hours. Suggestions?

[N2013]

2013 Yamaha 115, 570hrs. Did an oil change earlier this year, ran the boat for a week back in Jan during that unreal week. Checked earlier this week and now my oil looks like chocolate milk. Engine oil was dark when I changed earlier this year, not milky at all. Thermostat was replaced Nov 2024, but I pulled it, cleaned & tested the thermostat. Opened and closed fine unfortunately. Did a dig online and Is condensation capable of turning oil that colour? The pic shows the engine oil that is more white than the majority of the oil (last dribble of drained oil). Engine gets up to temp when running. I hate to think worse case scenario. Do I need a compression test to check head gaskets? I’ve heard condensation can form inside the engine if it’s not getting up to temp/not ran enough during the winter. It’s been really humid/wet and cold. Have used a motor this winter as well during storage.

I’m either gonna spend $300 for a new thermo and a couple oil changes and test it for myself and hope all goes well. Or do I pay for a shop to test/diagnose?

Lots of gremlins so far. I’m off to an expensive start this year.

 

[Hardy Guy]

I would go with condensation. Change thermostat or not. Keep an eye on it before you go crazy.

 

[seachicken]

I would change the oil and try it again however I think it's going to be more than that. Condensation builds slowly especially with heating and cooling cycles that don't get fully up to temperature to dissipate moisture. It doesn't take much to get the chocolate milkshake, I experienced it once in a gearbox and it was a small percentage (want to say 50 or 100ml in 20L?) once we had oil analysis done. But it is still generally more than from condensation. Sorry this sucks.

 

[InterMechanico]

You can try sending out an oil sample for analysis. Look for water content (will be positive) as well as sodium (if it’s salt water)

If it’s salt water, there is likely an engine issue such as head gasket, etc

 

[scott craven]

Primary cause of milky engine oil in a 4 stroke outboard is a blown head gasket.
Not good but repairable.

 

[Doug]

Maybe the thermostat is stuck open and the engine is not heating up to burn off the internal condensation. Maybe take the thermostat out and do a hot water test, and see if it opens and closes

If the last thing you did prior to having the problem was the thermostat, I would check the thermostat first with new gaskets. If you get water in the oil again $$$.

I hope it is a minor repair.
Good luck.

 

[Samnjoe]

Change the oil. Run it for 1/2 hour. Check it again.

 

[N2013]

Maybe the thermostat is stuck open and the engine is not heating up to burn off the internal condensation. Maybe take the thermostat out and do a hot water test, and see if it opens and closes

If the last thing you did prior to having the problem was the thermostat, I would check the thermostat first with new gaskets. If you get water in the oil again $$$.

I hope it is a minor repair.
Good luck.

I tested the thermostat and it opened and closed fine. Only 1 season on it. The thermostat housing didn’t have a gasket. Nothing shown on the Yammy parts site either. The engine has been operating fine (no overheating or poor performance) so I’m optimistic it’s condensation. I’m not getting it up over 1500 rpm heading out to the fishing ground as the bay is right out front and it has been a cold and wet season. I’m gonna run a couple oil changes through and will replace the thermostat. If I get water in the oil again I’ll bring the boat in to the shop and grab my ankles.

 

[scott craven]

Good plan, I would add that it may be an idea to take it for a good rip after the oil change.
Run it up to mid range rpms for 15 mins or so.
You want the motor to heat up and burn off any condensate.