Water in lower unit oil (Causes & Solution

Water in lower unit oil! A damaged driveshaft seal, worn-out prop shaft seal, and defective gaskets on the oil plugs are the culprits that cause the water to get in the lower unit. By addressing these problems and solving them, you can easily overcome this water pump issue. Fortunately, we will break down every culprit behind this problem. So, stay in tune with us till the end to explore the solution.

3 Causes of Water In Lower Unit Oil And Their Solutions

This chapter will reveal everything that causes water to get into the lower unit and mix with the oil.

  1. Cracked Driveshaft Seal 

A damaged driveshaft seal is the main culprit that allows water to seep into and mix with the oil. It operates under more pressure than others and gets easily cracked. A small hairline crack on the seal could allow water to enter. Besides, a leak in the driveshaft housing o-ring can also be the caveat behind this problem.

Water in lower unit oil
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Replacing the damaged driveshaft seal will be a simple fix to this problem. You can find the driveshaft seal between the water pump and the lower unit. So, pull out the leaky seal from there and replace it with a new driveshaft seal.

We recommend you get a replacement water pump housing kit like this one that features everything you need.

If you don’t know how to replace the seal, you can check out this tutorial. Alternatively, you can hire a certified mechanic to avoid the replacement hassle.

  1. Prop Shaft Seal Gets Defective

Our second suspicion is the prop shaft seal. If you don’t maintain the prop for a long time, it’s possible a fishing line and other stuff may wrap around the prop. And guess what, the fishing lines can easily cut the prop shaft seal and leak it.


Start with taking the prop shaft out and inspecting it for a wrapped fishing line. If you find the seal of the prop shaft gets damaged, replace it with a new one. This tutorial will come in handy if you don’t know how to replace the prop shaft seal.

  1. Worn-out Vent or Fill-plug Gaskets 

Most of the time we overlook the gaskets on the vent & fill plugs. But they can also be the culprit. If those gaskets wear out or get damaged somehow, water can easily go into the lower unit of the water pump.


Just take a look at those gaskets on the oil plugs. If they get frayed out, replace them with new ones. Also, don’t hesitate to replace the oil plug if you find the gaskets come off and stick in the plug hole. You don’t want to take the risk of getting water in the lower unit, do you?


What does the water in the lower unit mean?

Water in the lower unit indicates the seals on the driveshaft get cracked or damaged. That’s why the water makes its way to enter the lower unit.

Why is my lower unit oil white?

The lower unit oil gets white if water enters the lower unit and mixes with the oil. If you find the lower unit oil white, understand the seals of the water pump get damaged. So, replace the leaky seals right off the bat to avoid further damage to the pump.

How do I test my lower unit oil for water?

Testing the lower unit oil for water is simple. If the lower unit oil turns white or milky, understand that water intrusion is the culprit behind this.

Can you fill lower unit oil from the top?

No, you can’t fill lower unit oil from the top. It is because of the air bubbles that don’t allow it to get completely filled. When trying to add oil from the top, it will create an air pocket and prevent the fuel from filling the lower unit fully.

How often should you change your lower unit oil?

Outboard manufacturers recommend you change the lower unit oil once a year or every 100 hours of operation.

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Water can get in the lower unit due to a damaged driveshaft seal or the leaky prop shaft seal. What we suggest is- to check the driveshaft seal as it’s the main culprit behind this. If you find it worn out, replace it immediately.

Next, inspect the prop shaft as fine debris or fishing line may wrap it up over time and damage its seal by cutting.

Lastly, check out the gaskets on the oil plugs and replace them if needed. We recommend you check these seals every time you change the oil. It will help you in the long run.

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