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Why Is Oil Leaking From Lower Unit Weep Hole of Your Boat? – Mechanical Pump Seal Failure

Oil Leaking From Lower Unit Weep Hole

Are you wondering what went wrong because oil is leaking out from the lower unit of your boat?

Well, just so you know, lots of other boat owners face this problem and it’s quite common.

So, why exactly is oil leaking from the lower unit weep hole of your boat?

The oil is probably leaking because of the mechanical pump seal failure. Another reason might be a bung failure.

Then leak in the input shaft can also lead to this problem. Lastly, you might be going through combustion or 2-stroke oil leaking.

We’ve written in detail about how to detect what’s causing the oil leak and how to fix this. Do check this out so you can solve it.

Causes & Solution of Oil Leaking out of Weep Hole

Why Is Oil Leaking From Lower Unit Weep Hole of Your Boat

Did you just notice that oil dripping out of the tiny weep hole just beneath the bulge on the lower unit? Let’s see why this might be happening.

Water Pump Seal Failure

First of all, the water pump seal is no longer sealed. This is why the oil is leaking out. It’s possible that all you need is a new water pump.

The water pump seal fails because of heat caused by friction. Lack of lubrication leads to friction.

So, are you by any chance riding a Yamaha boat? Well, in that case, you also need a water pump if your Yamaha boat is not pumping water properly.


The solution is always to do a pressure test. Aspray of soapy solution will reveal any leaky o-rings.

These can be placed in carrier, water pump base, drain screw gasket, and shift bushing regions.

A pressure test may be performed at your local repair shop. This will provide a definite response to the query.

You may also unbolt the lower unit so you don’t have to drag the entire boat there. Simply hand the unit over to the experts for pressure testing.

Leak in the Input Shaft

Leak in the Input Shaft

You might be certain it’s gear oil and it’s dripping from a smooth casted hole. In this case you probably have a leak in the input shaft or shifter shaft.

Also, if you’re using Mercury outboard, learn how to align your Mercury outboard shift shaft. If you do not do this properly, the gear oil might leak out.


In case of a leak, drain and remove the lower. Discard the top water pump cover to reveal the seal. Then pump roughly 10 psi into the lower and inspect for leaks. Regrind the shaft if you cannot seal it with emery tape.

Combustion Oil or 2 Stroke Oil Leaking

Now, oil leaking out might not be a huge deal if it’s combustion oil. Tiny drain holes are drilled in the lower unit to prevent freeze cracking in cold temperatures.

If the oil is bluish-green, it’s 2 stroke oil. In that case your bottom cowling is most likely leaking. Make sure there isn’t any oil pooling at the bottom of the cowling.

Combustion oil always flows out of the bottom. It’s only natural and that’s what you’re experiencing.

Stroke Oil Leaking


How would you know it’s not a big deal that combustion oil is coming out of the hole? Try vacuum testing the hole.

You can go boating if it sustains a vacuum test!

How do you do a vacuum test? It can be easily done. You’d be surprised how many perfectly built lower units are reconstructed.

That’s because someone notices aerated oil and believes it’s because of water infiltration.

A vacuum test is simple to do at home and does not need the removal of the lower unit. A vacuum pump with a gauge is required for this.

You might be able to borrow one from where you borrow your car tools too.

The drain plug has a 3/8 UNC thread. So, you’ll need to convert a 1/8-inch pipe to a 1/4-inch pointed fitting for your test fitting.

Thread one end to 3/8 UNC and connect the other end to a suction hose. Start by draining the lower unit first.

Next, lift up the lower unit as much as possible to avoid sucking the oil out.

Also, check your boat’s spark plug and replace it if it’s defective. You can take help from a professional on this.

Also, make sure you choose a good spark plug. Because if the spark plug is damaged, the engine oil might leak out.

Bung Failure

Now, the oil might also be leaking out of the fill bung. The threads must be worn out or not aligned properly. Another reason for this issue is that the bung got loosened.


If that’s the reason behind the oil leakage, you’ll need to tighten the bung. If the leak remains, you’ll need a new gasket or O ring. Because your O-ring is at fault.

Now, it can be either the lubricant leaking out or your o-ring is damaged. In order to know what’s wrong, try to do a pressure test.

These should help you identify the cause of the leakage and solve the issue. After the pressure and vacuum test, you can enjoy riding in your boat again.

But this time make sure to ride the boat safely to keep both yourself and your boat safe!


Stroke Oil Leaking

Why is oil coming out of the hole in the lower unit outboard?

Because oil is leaking from it, the driveshaft seals (which are quite frequent) are worn and will leak oil when the motor is tilted up and down.

Because the seals are located beneath the water pump plate and in the water route, any oil leakage will migrate (pun intended) to the drain hole, which is the lowest position.

What is the little hole next to the top drain plug on the lower unit of the boat motor?

When you take the boat out of the water, there is a drain hole for any standing water on top of the bottom section. It drains the water out.

Can you overfill the lower unit oil?

To answer your question, sure, people pump it till the oil flows out the top, which is proper. But then put the top screw in and give it a couple more pumps just to be safe.

Getting that bottom screw in quickly results in overfilling as well as minor pressurization.

oil coming out of the hole in the lower unit outboard

How do you stop an oil seal from leaking?

One option is to simply replace the seal. This is usually the best option if the seal is old or damaged. Another option is to use a sealant designed for oil seals.

This can be applied to the outside of the seal to help it hold in oil better.

If neither of these options works, or if you can’t get a new seal or sealant, you may need to resort to using RTV silicone or another type of sealant around the outside of the seal.

This will not be as effective as a proper replacement or sealant, but it may be enough to stop the leak until you can get a proper fix.


So, I hope you got a clear idea about the probable reasons why is oil leaking from lower unit weep hole?

Also, you’ve learned what to do when this issue is bugging you. Pressure test and vacuum test, both can be done by an expert.

Get some professional help if you cannot do it by yourself.

Let us know how you fixed it! Also, if you have any other suggestions, contact us. Take care!

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