Why is There Milky Oil in My Outboard?

Oil needs to stay fresh to do its job, which is lubricate the moving parts of an outboard engine to prevent wearing and overheating. If your outboard's oil is milky or cloudy, something is definitely wrong.

Milky oil in outbaord engine

Milky oil coming out of your outboard is a sign of water intrusion (oil mixing with water), which is a critical problem that needs to be addressed immediately. So what causes engine oil and gearcase oil to look like a chocolate milkshake? Here are four common causes of milky outboard oil.

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Thermostat Stays Open

Milky oil can be caused by something as simple as a bad thermostat. If the thermostat is stuck open, it’s more than likely damaged and needs to be changed out. Thermostats are cheap and easy to replace, so check there first before you go deeper into the engine.

Milky oil in outboard causes bad thermostat

When a thermostat is stuck open, it allows water to mix with the oil through condensation created by the constant heating and cooling of the engine. The condensation gets past the piston rings and into the cylinder walls, resulting in milky oil. In other words, if the thermostat doesn’t do its job, water gets into the oil. A working thermostat opens and closes when a certain temperature is reached to let hot water out and allow cool water to pass through. That means a broken or missing thermostat disrupts ideal combustion temperature. Best case scenario, changing out the thermostat could solve the milky oil problem.

Damaged Piston Rings

So what if your milky oil problem is not the thermostat, but something more serious? The next thing to check is the piston rings, which will allow oil into the combustion chamber if they’re damaged. You’ll see milky oil as a result of fuel mixing with the oil because the piston rings are damaged or worn and aren’t sealing properly, resulting in oil entering the combustion chamber. 

Milky oil in outboard causes piston ring damage

Blown Head Gasket

Another critical cause of milky oil is a blown head gasket. When a gasket is damaged or worn, it doesn’t seal properly, allowing water to get into the head. Water gets sucked into the combustion chamber when the pistons are in motion, creating all kinds of chaos within the engine and producing milky oil once it gets past the piston rings. 

Milky outboard oil causes blown head gasket

Damaged Cylinder Walls

Scored cylinder walls allow small amounts of oil, fuel and exhaust to pass through the combustion chamber, which could also lead to milky oil. Cylinder wall damage is usually caused by layers or pieces of carbon in the engine flaking off and getting caught between the piston and cylinder wall, wreaking havoc on the engine. If this is the cause of your engine’s milky oil, you have your work cut out for you.  

Milky oil in outboard causes scored cylinder walls

Aside from a bad thermostat, any of the other causes for milky outboard oil listed above are severe and therefore incredibly costly, and you could end up having to rebuild the engine.

How to Flush Out Milky Outboard Oil 

Regardless of what’s causing milky oil, you need to flush that contaminated oil out of the outboard. Here are a handful of simple things you can do to flush out milky oil.

Milky outboard gearcase oil

  • Wear latex gloves and have a drain pan ready for the messy spill flushing out the oil will cause.
  • Open the oil drain plug with a socket, then add in a non-solvent engine flush additive to break down carbon deposits and gunk in the oil.
  • Warm up the engine with the flush still inside to allow the oil to drain out faster.
  • Turn on the engine and let it run for 5 to 15 minutes (depending on the size of the motor).
  • Place a drain pan under the outboard’s oil drain plug, then remove the plug to let the oil drain out completely.

Draining milky outboard oil

Change out the oil filter, then fill up the engine with new oil, per the manufacturer’s specifications. Allow the engine run after the oil flush and change to see if you still have milky oil. If you do, flush out the engine and change the oil out one more time to remove any leftover impurities.  

Regular maintenance helps prevent your outboard’s oil from getting contaminated with water. It’s recommended servicing your outboard after every 100 hours of use to keep it healthy. Watch the video above to see how to perform a 100-hour service on an outboard engine



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