How to Troubleshoot an Outboard Lower Unit

Lower unit problems on an outboard can start off small and snowball into a debilitating mechanical issue that stops your boat cold. 

Identifying problems early cuts repair costs, and in some cases can keep you from having to replace the lower unit altogether. Here are some outboard lower unit troubleshooting tips and videos to get you started.

Grinding Noise in Lower Unit

You try to engage your outboard in either forward or reverse, and you hear some grinding, but the unit does engage. While the sound might be originating in your lower unit, the problem might not. 

Outboard lower unit clutch dog

The clutch dog circled in the image above engages to allow the outboard to run in either forward or reverse. When that engagement isn't smooth, you'll hear grinding. This noise can be a sign that you need to adjust your throttle cable, so that the shift is completed before throttle is applied and RPMs increase.  

Check out the video above for information on how to do maintenance on the throttle cable of your outboard to correct the problem.

Milky Lower Unit Oil

Changing the lower unit oil should be a regular part of your outboard maintenance schedule. 

The oil that comes out as you drain the lower unit can tell you a lot about what's going on inside the gearcase. Milky lower unit oil means you have water in the oil, which means you've lost a seal somewhere. The first place to look is the output seals, which are found behind the propeller. They're commonly damaged when fishing line gets wrapped around the prop shaft, and the seal can be inspected by removing the prop. 

Usually there are two output seals placed one after the other as a redundancy, so this is the first place to look. 

Outboard prop shaft seals

If there's no leak there, you'll have to get more involved. Watch the video below on replacing the water pump on the Yamaha F225 to see how to remove the lower unit and water pump. 

Next, check out our video on how to replace the input seals on your outboard to see how to dig down to the input seals. 

If a visual inspection of the input and output seals doesn't give you answers, your next step is a lower unit pressure test. 

Use a gearcase pressure tester to build up pressure in the unit and spray the seals with some soapy water to reveal leaks that you can't see.

The next step would be to check the output shaft carrier seals, shown below. 

Outboard shaft output carrier seals

This involves pulling the carrier out of the lower unit. Watch our Yamaha outboard lower unit disassembly video below to see how to get that done.

Outboard Engine Overheating Problems

If your engine is overheating, the first place to look is the impeller inside the water pump. This pushes water from the lower unit intakes up to the engine. 

Damaged outboard water pump impeller

To look at it, you'll need to remove the lower unit and the water pump housing. Watch our video on how to replace the water pump on an outboard (featured earlier in this article) to get this done. 

Separating Lower Unit from Powerhead

When you reinstall a lower unit, it's important to re-grease splines. However, if you're trying to pull a lower unit, that ship has sailed. The best advice is to gently pry between the lower unit housing and the powerhead. Thin metal scrapers can be used just to get enough separation where you can then insert a screwdriver and pry against the scrapers. This will protect the mating surfaces between the powerhead and lower unit.

Need a little more help with troubleshooting the lower unit? Watch the video above to learn how an outboard's lower unit works

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