You checked the oil in your 4-stroke outboard and are about to tear into it because the oil is milky, right?
Not so fast!
While there might indeed be a costly reason as to why your oil is less than appealing, there might also be a very simple and cheap fix to solve this milky mess.
An outboard will have milky oil most commonly due to the thermostat being stuck open or being missing altogether. After the thermostat is ruled out, the cylinder walls might be scored, the piston rings damaged, or the head gasket is blown.
That’s the quick and easy answer for you. If you’d like to find out why, please continue with me and hopefully we’ll get you pointed in the right direction to get you fixed!
Stuck Open or Missing Thermostat
A stuck or missing thermostat will cause condensation to occur inside your cylinder walls due to the constant extremes of heat from the engine and cold water from the water pump. The condensed water will work its way passed the piston rings and into your oil which will make it milky.
Let me explain why.
Having a stuck or open thermostat result in milky oil is perplexing at first glance. Think of it like referred back pain. You might have a very tight hamstring in your leg that torques on your pelvis and causes pain in between your shoulder blades from the compensation.
Fixing between the shoulder blades isn’t going to solve the problem, and either is simply changing your oil.
When your outboard is running, the water pump in the lower leg is sending up water to cool your engine. When you first start your outboard, this water is mainly supposed to exit the tell tale, or indicator hole (which is that stream of water coming from your engine).
It is supposed to only exit that hole at first because the thermostat in your engine is actually trapping the majority of the water and the pump is simply keeping the pressure. The water is kept trapped in the galleries and water jackets around your cylinders until they get up to running temperatures. If the water was constantly replaced with cold water from the beginning, the engine would always run cold.
A cold engine is inefficient, as it makes combustion harder since the gasoline is less volatile and the air is more condensed. You’re essentially running lean at first.
The trapped water is supposed to heat up by cooling the engine as it warms up. When the temperature gets to a predetermined temp on the thermostat, the thermostat will open and allow hot water to flow out through your exhaust and allow new water in to help cool the engine and keep it at the ideal temperature for combustion.
When your thermostat is stuck open or missing, the engine will always have a very difficult time coming up to an ideal temperature for combustion. The cylinders will be trying to get hot but will constantly be cooled by new, cold water.
This is where we get problems with condensation.
You’ve got hot cylinders internally, and a very cold environment around them causing the cylinder walls to become cold despite the combustion internally. The extreme difference in temperature causes condensation from any moisture in the air to form on the inside of the cylinder walls like you would find if you brought a cold can of beer out of the fridge on a 90-degree day.
The condensation will glides down passed the piston rings and into your oil with each stroke of the engine.
The end result is that you have milky oil due to water being mixed into it.
Luckily, thermostats are cheap and this should be an easy fix. Simply install, and change your oil.
Scored Cylinder Wall
Hopefully you don’t have this issue from a piece of dislodged burnt carbon from inside your combustion chamber. It can happen though.
If your engine has been running rich, carbon will build up inside the combustion chamber. It will basically form into a hard layer of charcoal.
If a piece of the carbon layer flakes off, it can wedge itself between the moving piston and the cylinder wall and simply score a gouge in the steel.
Now, that path allows oil to travel up into your combustion chamber and bypass the rings, and it also allows fuel and exhaust to blow passed the piston and into the oil which can turn it milky.
Piston Rings Damaged
The same concept as above applies to a piston ring being damaged. If it is not sealing properly, then it is also allowing oil to enter the combustion chamber, but it is also allowing fuel and exhaust to blow back into the oil and can turn it milky.
Blown Head Gasket
Finally, you might also have a blown head gasket that ruptures from the combustion chamber itself out to the cooling water jackets that surround the cylinders.
With a break of this gasket, every time the piston descends, it will suck in water into the combustion chamber which will work its way down passed the piston rings and into your oil.
Conversely, when the piston drives up, it will expel exhaust through the hole in the gasket and into the cooling water jackets. This sudden influx of hot gas into the cooling water will cause the water coming out of your indicator to start to stream erratically as air is being blown back through the line.