How to Detect an Outboard Blown Lower Unit? (5 Symptoms)

The lower unit houses some of the most crucial components of the outboard motor. These include the gearbox, various shafts, and the water pump. A bad or gone unit means your boat won’t propel. Plus, your engine might suffer irreversible damage. 

To prevent the chain of negative events, you must detect the symptoms of a blown lower unit at the earliest. These include an erratic motor, a heated engine, strange noises, etc. Leaks and diminished performance can also be present. 

Now, you might have experienced some of these symptoms in your outboard. But you are still in doubt about whether to blame the lower unit. Learn about them in more detail and confirm your diagnosis. 

How Do You Know When Your Lower Unit Goes Out? 

The lower unit will start showing signs when it’s blown out or about to be soon. If you are careful enough to detect these symptoms early, you can prevent the faulty unit from affecting other parts of your outboard.

Symptom 1: Locked Outboard Motor

When the lower unit fails, you will often find that your Outboard motor is locked or reluctant to propel. This behavior of the motor can feel somewhat erratic. 

It usually indicates damage to one of these two contents in your lower unit- shift shaft and prop shaft.

The shift shafts control two things. It puts your Outboard motor either in or out of gear. The shaft’s relation with the transmission makes this possible. 

It also determines the direction of your propeller’s rotation. So, to properly start and move the boat forward/backward, the outboard motor needs the shift shaft to be in good health. 

Another reason for the motor to lock up is a faulty prop shaft. The propeller shaft connects to the pinion bearings and gains horizontal rotational power from the gear case. 

As a result, the propeller rotates. When the shaft’s damaged, the propeller will lock up. 

Symptom 2: Engine Not Cooling Down

A lower unit failure in the Outboard Motor can readily affect the powerhead. Among them, a heated engine is the most important. However, this can also happen if the engine is very old. 

An engine heating up frequently from lower unit failure can stop working soon. The heated engine will also damage other parts of the powerhead. Such as the pistons and cylinders. 

The reason for this is the water pump in the lower unit. This pump propels up water throughout the engine to keep it cool. 

If damaged impeller blades are inside the pump, it will fail to suck the water outside the boat. 

On different occasions, the fault might be within the drive shaft of the lower unit. 

The driveshaft rotates the impeller blades and helps the motor draw water in the first place. So, if the drive shaft has worn out, the pump would fail to keep the engine cool. 

Symptom 3: A leaking Unit

If oil or similar fluids are dripping from your outboard motor, you most likely have a blown lower unit.

All the shafts and the gearbox use oil and lubricants for smooth movement. Various seals and gaskets keep these oils confined to where they are needed. However, sometimes these seals can break and cause leakage. 

There are three seals and two gaskets in the lower unit. These are the driveshaft, prop shaft, and shift shaft oil seals. And the O-rings of the shift shaft plate and gearbox bearing housing. 

Among them, the prop shaft oil seal is the most exposed. So, it is the commonest seal to damage. This is the first seal you should check when you have a leaky outboard.

Unlike the prop shaft oil seal, the other seals and gaskets are mostly out of sight. 

However, if you notice a grey sludge in your outboard during a lube change, you can correctly assume that the O-ring in the gearbox has broken. Because this sludge appears when water and gear lube mix together. 

Symptom 4: Outboard Losing Power

Your outboard might be losing power, despite a good source and engine. You can consider a failing lower unit in that case. Most probably, the gearbox has blown out. 

The drive shaft brings vertical rotational power from the engine to the gearbox. The gears (forward, reverse, pinion) and the dog clutch translate it into horizontal prop shaft rotation.

Over time, the gear shims wear out. As a result, the exchange between the drive and prop shaft becomes weak. To the driver, it feels like the outboard is losing power. 

Symptom 5: Abnormal Noises

A bad gearbox can also be variably noisy. Sometimes you might hear a clunking in your outboard. It indicates some part has fallen off the motor assembly. 

In most cases, it’s the tooth of some old gear. The piece can easily stick between the working gears and bring your whole outboard to a stop. 

Another symptom is hearing the outboard grinding. The noise comes during gear changes. In this case, the dog clutch probably has worn-out teeth. 

The clutch is the connection between the shift and prop shaft. The shift engages the clutch with Forward and Reverse gears. In turn, the propeller changes its direction. 

Changing gears at high RPM causes the dog clutch to wear out quickly. Another cause is correct rigging. 

The engine will get the wrong input if the outboard’s cable connections are bad. This will put the drive and prop shaft out of sync and stress the dog clutch. 

Apart from these, your gearbox will constantly make a chattering noise. It indicates the gears are old and loose.

7 Possible Causes of Lower Unit Failure On Outboard?

The lower unit has your boat’s propeller. So, a damaged lower unit means your boat isn’t moving. 

However, if you are aware of the reasons behind the unit’s failure, you might be able to prevent the situation. Let’s have a look- 

  1. Collision with submerged rocks or reefs 
  2. Driving into a sandbar 
  3. Propeller striking floating objects 
  4. Marine life Clogging the Water Pump Impeller
  5. Insufficient shaft and gear oils 
  6. Not changing gear lube regularly 
  7. Rust and corrosion in the internals (water entry)

How Much Does it Cost to Rebuild a Lower Unit?

A failing lower unit doesn’t always mean you have to buy a new outboard. You can replace the failing parts and rebuild your lower unit. 

If you do it correctly, your outboard will again run properly and may last its normal course. Let’s have a look at the associated costs. 

Part to be replacedPrice (labor charge + parts’ costs)Comments
Prop shaft400-500$If you aren’t an experienced mechanic, take the help of one for this replacement. Because assembling the outboard wrong will not help your issue.
Gear Box800-1000$When you have a chattering gearbox, it is best to replace the whole thing. Otherwise, you will have to keep on visiting the service center. 
Oil Changes160-190$This is something you can do yourself. In that case, your costs will be around 20-30$ (oil+filter price). Here’s a video for guidance.
Lube change150-160$.You can do it yourself at the reduced cost of 10-20$. It is easier than an oil change. Plus, a small amount of lube shall suffice. 
Water Pump Rebuilding300-400$Experts recommend changing your pump impellers every 200 hours. 
Total Outboard Rebuild1500-4500$Prices vary according to models. 

Tips for Maintaining the Outboard Lower Unit for Long-Lasting Use

Most outboard lower unit contents are designed to last long. So, the reason why many boaters see these units failing way before their time is poor maintenance. Follow these tips to take better care of your outboard lower unit – 

  • Change Gear lube every 100 hours
  • Regularly inspect your drain screw magnet for excessive metal particles
  • Don’t change gears on High RPM.
  • Practice safe driving, such as avoiding collisions and running aground.
  • Maintain clean and well-adjusted propellers. 
  • Routinely check for obstruction in the cooling passages and impeller blade and take action. 
  • Properly winterizing your boat during cold months 
  • Ensure grease points are not freezing.
  • Always check for cracks and broken seals and mend them at the earliest. 


Hopefully, you now have sufficient knowledge to detect and solve outboard lower unit problems. There still can be many things left unanswered. Here are some frequently asked questions that you might want to know.

Can you run an outboard without a lower unit?

The propeller is part of the lower unit. So, without the unit, the boat won’t drive. But it is possible to test-run the motor minus the unit (during inspection & servicing). Here, use a hose system to make up for the cooling function of the water pump. Otherwise, your engine might be gone forever.

What happens if I keep driving with a damaged propeller?

Firstly, your boating experience will be very unpleasant. A boat with a damaged vehicle will vibrate and jerk uncontrollably. At the same time, it will affect other parts of the outboard motor and result in costly repairs. 

How often should you service an outboard motor lower unit?

You should inspect and service your outboard unit after every hundred hours of use. For most boaters, that translates to once a year. Take this opportunity to change gear oil, retouch grease points and change worn-out seals and gaskets. 

What should I look for when buying a new outboard?

First of all, buy your outboard engine from a renowned brand. This ensures warranty and convenience of future servicing. Secondly, ensure it’s compatible with your boat and has adequate horsepower. Also, outboards with more cylinders and longer shaft lengths perform the best.

Final Words

In conclusion, detecting the symptoms of a blown lower unit in your outboard motor is crucial to prevent further damage and costly repairs. If you notice any of the signs mentioned in this article, such as a locked motor, engine overheating, leakage, power loss, or abnormal noises, it’s time to investigate the lower unit. Regular maintenance and inspections can also help prevent lower unit failure and ensure smooth and safe boating experiences. Remember to always prioritize safety and maintenance to enjoy your time on the water to the fullest.

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