Signs of a Bad Outboard Lower Unit & What to Do Next

On vessels with an outboard, it’s essential to keep debris free of the prop, as well as performing other vital maintenance. Otherwise, you could end up with a bad outboard lower unit.

With three distinct sections of the outboard motor, it’s essential to determine when something has gone wrong with the lower unit or one of the other areas.

Bad outboard lower units can give the operator shifting issues, and other signs will include water in the gear lube, metallic particles on the drain screw magnet, clunking sounds when shifted, or the loss of the ability to shift into gears.

We will look closer at the importance of the lower unit, how it works, symptoms that something might be wrong and some general outboard maintenance tips.

bad outboard lower unit

What is the Lower Unit on an Outboard Motor?

The lower unit of the outboard motor is otherwise referred to as the foot, gear case, or gear box. It’s the bottom portion that houses the gear case and prop shaft that changes the direction that the propeller spins in.

To keep your outboard running its best and give it the longest life possible, you must take care of the lower unit.

Performing lower unit maintenance is as simple as keeping the debris and fishing line free from the prop, as well as regularly changing the oil. We have an in-depth extensive article on what the different colors of gear lube tell you when you change it here.

How Does a Lower Unit Work?

The lower unit is also known as the gearcase. It’s the portion of the outboard that receives the rotation and power from the engine’s powerhead to transfer it to the propeller and prop shaft.

The propeller rotates, which allows your boat to move either forward or backwards through the water.

If one part of the lower unit isn’t working correctly, or fails, you could become stranded in the water. That’s why it’s vital to understand how each part works and how to maintain it.

Driveshaft: How it Works & Maintenance

The driveshaft is responsible for converting the rotation power from the motor to the gear case.

You don’t need to perform much maintenance to the driveshaft, as long as the lower unit remains maintained.

Water Pump: How it Works & Maintenance

The water pump delivers water through the engine to cool it. This part uses an impeller that is attached to the driveshaft, using the rotation as power.

When the drive shaft rotates, the impeller inside the water pump turns. The blade’s movement sucks water from below the water pump housing housing and pushes it into the engine.

Once the water passes through the engine, it is jettisoned out, back to where it originated.

When the water pump fails, the motor overheats. Left unrepaired, it can cause worn-out pistons or cylinders, eventually causing the engine to seize.

To maintain the water pump, you want to disassemble and inspect it. Replace the impeller once every 300 hours or once every two years. You might also need to replace the bottom plate, cartridge, O-rings and gaskets when you change the impeller.

Shift Shaft: How it Works & Maintenance

The shift shaft is responsible for putting the outboard in or out of gear by working with the transmission. It’s also used to decide which direction the prop shaft needs to rotate, ensuring that the boat goes in reverse or forward.

You won’t have to maintain the shift shaft in any way, as it should last as long as your outboard.

Gearbox: How it Works & Maintenance

The gearbox or gear case uses the vertical rotational power coming from the driveshaft and turns it into horizontal rotational power for the prop shaft and propeller.

It also helps to determine which way the propeller and prop shaft should rotate.

The drive shaft comes into the gearbox, but it ends when it reaches the pinion gear. This pinion gear is engaged with both the forward and reverse gears.

When the drive shaft rotates, the pinion gear also rotates, allowing the gears to turn.

There is also a dog clutch sitting between the gears and attached to a shift shaft. The prop shaft is also attached to this dog clutch.

Anytime that that shift shaft is placed in Neutral, the clutch doesn’t engage with the gears, keeping the boat motionless.

When the shift shaft is in Forward, the dog clutch is engaged, causing the prop shaft and dog clutch to rotate in a specific direction. The opposite occurs when the shift shaft is in Reverse, allowing it to move the other direction.

Both gears and the prop shaft are mounted with bearings, which keeps everything rotating freely, yet supported in the gear case.

As time goes on, the dog clutch teeth wear out, so you will need to replace it. When it’s time for a replacement, you might hear a grinding noise while shifting.

You can prevent premature clutch wear by shifting at lower RPM. You also want to maintain the rigging, ensuring none of the shift cables or throttle cables get stretched.

Additionally, the gear shims and pinion will wear down. When this occurs, power transfer between the proper shaft and driveshaft is reduced, so the outboard feels like it has lost performance.

Additionally, the gearbox can become noisy or the gears might chatter as they become loose.

Oil Seals: How it Works & Maintenance

The oil must remain within the gearbox with the help of O-rings and seals. These also keep water from getting into the gear case.

Some of the seals on your outboard’s lower unit include:

  • Shift oil seal and shift shaft plate O-ring
  • Driveshaft oil seal
  • Gearbox bearing housing O-rings
  • Prop shaft oil seal

The prop shaft oil seal remains exposed, making it more susceptible to damage. Because of this, you want to inspect the seal often and replace it at the first sign of leakage or wear.

As far as the other O-rings and seals, they aren’t easily seen. However, you can tell when one of the internal parts fails because the water will begin to mix with gear lube, leaving a gray sludge. When this happens, change the gear lube.

You should also change this lube after 100 hours on most outboards. If you notice that water has mixed, it’s time to replace the faulty O-ring or seal. You will be able to tell which seal is leaking by performing what is called a pressure test.

How Do I Know if My Outboard Lower Unit is Bad?

outboard motor lower unit is bad

Numerous things can occur when you have a bad outboard lower unit, but here are the most common symptoms.

Water in Gear Case Lube

As previously discussed, once water mixes with the gearbox lube, you know you have a problem. When you change the lube, you can look for signs of trouble.

Which is why it is so important to change your gear lube every 100 hours. This way you can catch the issue and fix the leak.

Before the water in the gear lube destroys the lower unit.

Metal Contaminants on Gear Case Drain Screw Magnet

There should never be more than 3/4″ of metal particles on your lower magnetic gearbox drain screw magnet. When you are examining the magnet, mash any of the contaminants in your hands.

If it turns into a gray powder, you can completely ignore it. Otherwise, if you feel metal in the process, you should be concerned. A small amount of metal is almost always going to be on the magnet.

It’s just once the build up of metal becomes a massive amount that you will need to be concerned and begin the process to figure out what is going on with the lower unit.

Clanking Sounds

If you begin hearing steady clanking noises, it could be the pinion or gears. When a tooth is lost, it will turn into this annoying sound. And at the right RPM, will lodge between the gears and blow up the lower unit!

How Do You Put a Lower Unit in Neutral?

If you have removed the lower unit and bumped it, you might have to re-adjust for Neutral if the shift linkage isn’t lined up.

In most cases, you simply need to rotate the spline shaft to align the shifting again. Grab it with a set of pliers and rotate it until you feel it snap back into Neutral.

How to Change Your Lower Unit Oil

One of the most essential maintenance tasks for your outboard lower unit is to change the oil. Before beginning, you want to read the owner’s manual to get the appropriate weight and type of gear lube to use.

The gear lube should be changed every 100 hours or once per year.

Equipment you will need:

  • Flat-head screwdriver
  • Oil drain pan
  • Cardboard to catch any drips
  • Lower unit oil pump
  • Quart of gear oil

Step #1: Position Motor

Put the outboard in a vertical position. The prop shaft should remain parallel to the ground. If the drain seals are in the back of the lower unit.

If they are on the bottom of the nose of the gearcase. Then you want to tilt the engine or lower unit up to make the drain screw the lowest position on the gear case.

Place cardboard under the motor on the ground. Place the oil pan underneath the prop.

Step #2: Remove the Drain Plug

Use your screwdriver to remove the drain plug. With some outboards, you might require an impact driver, which reduces the chance of stripping threads.

The lower unit plugs are magnetic, so you might see some metal shavings, which indicate wear. If you only see a few, there’s nothing to worry about, but an excessive amount should cause concern.

Step #3: Remove the Top Plug

By removing this plug, you vent the lower unit, ensuring that the oil drains faster. On some outboards, you might find two of these plugs, requiring that both should be removed.

Don’t remove this plug before you take out the bottom drain plug or there will be a substantial mess.

Step #4: Inspect the Oil and Plugs

What color should lower unit gear oil be? Your oil should never be milky, but instead, look dark. If you notice that it’s milky, you probably have water contamination caused by a leaky seal or debris.

Don’t simply change the oil, but fix the problem first. Otherwise, water will continue to seep into the oil, leading to a damaged gear box.

Also, take a look at the gaskets on the oil plugs. If they are worn or frayed, it’s time to replace them. If your gasket comes off of the plug, remaining behind in the hole, it’s time to replace the entire plug.

Step #5: Refill the Lower Unit

Thread the lower unit pump tip into the bottom plug hole. Keep the upper plugs out during this time.

Gently pump, so air doesn’t get into the gear case. When oil begins to trickle from the top vent hole, you know that the lower unit is full. Stop pumping.

Step #6: Insert Top Vent Plugs

rebuild a bad outboard lower unit

Thread your top vent plug or plugs back into place and wipe them down.

Step #7: Insert Bottom Drain Plug

When you remove the pump nozzle, you want to quickly put the bottom drain plug back in.

Snug this plug according to the owner’s manual. It will likely require a specific torque.

Wipe it down and check for leaks.

Tilt the motor up to look for any oil leaks from these plugs. If there is any spilled oil in your area, you need to clean it up. Also, make sure to dispose of the oil properly.

Outboard Lower Unit Rebuild Cost

If you have a bad outboard lower unit, you have several options. You can rebuild the unit or purchase a new one.

If you are mechanical, it would cost you far less money to rebuild the lower unit than to purchase a new one. However, this is going to require more of your time than simply making a purchase.

You have to evaluate which is going to be better in your situation. Look up the price of a new outboard lower unit and compare that to the price of the parts for a rebuild.

Check Us Out!

Hopefully, this article helps you troubleshoot your outboard lower unit issues. We want to help you keep your boat running smoothly and efficiently. You can also find more tips and guidance by visiting our YouTube channel.

We also curate lots of blog articles to give you the boating information you need to enjoy your time on the water.

Brian Jones

Brian Jones has spent most of his career as an ASE Certified Master Tech. Today, he spends his time freelance writing and consulting. He lives outside of Dallas, TX with his family and loves anything mechanical.

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