Low Compression in One Cylinder Outboard: Cause and Fix

Riding on a boat with family or peers is a fun activity to do on a hot summer day. But the fun quickly gets spoiled when your boat starts to fickle or show poor performance. At times like this, when you get down to inspect your boat only to find low compression in one cylinder outboard motor.

Issues like this are caused for a bunch of reasons. Like- misplaced head gaskets, valve issues, damaged cylinder walls, broken piston rings, etc. Compression tends to get lower drastically when one or two cylinders start to leak for the damaged engine. 

And that’s not all as to why you can come across a cylinder low in compression. More factors come into play that causes this issue. So, keep reading this article if you want to know why compression lowers and how to fix it. That being said, let’s take a look. 

What Causes Low Compression in Outboard Motors? 

An outboard motor is the heart of marine propulsion. It’s a system for boats that uses an engine, gearbox, and propeller to function. The thrust a boat gets is solely possible for this system–an outboard motor. 

However, when issues get detected at any of these parts, the health of the outboard starts to deteriorate. That includes – low compression in the engine. 

The reason is, an outboard gas engine needs fuel, air, spark, unobstructed exhaust, and compression in the exact required amount. 

And any issue with these aspects can cause problems in the engine – causing problems in the outboard. 

Compression powers the engine that, in turn, powers your boat to run. And it works by letting the piston and rings compress the air-fuel mixture into the cylinder – igniting a spark. 

So, if you find your compression to be low in one or more cylinders, it’d mean your engine isn’t running well.  

In situations like these, a compression check can indicate if cylinders are working or not. A two-stroke cylinder engine should have a compression reading of 110-130PSI. 

The highest difference of the readings should be 10PSI of each other. Any rate below 90 is considered a low compression. 

So, it’s always a matter of concern when compression gets low. Since it indicates a problem, it might also point out other engine problems an outboard can have. And that’s why it’s necessary to know what causes this scenario. Only then can we find the issue.

Now, you might think, what possibly causes low compression in a cylinder? Well, there are a few common causes that drastically work in lowering the compression. They are:

1. Head Gasket Failure

A gasket is a tool that keeps everything together in the engine by preventing the coolant from getting mixed with the fuel. This tool is located between the cylinder head and cylinder wall. 

There’re a lot of causes that take down your cylinders by lowering their compression. And one of the common reasons is a broken head gasket. When it’s not aligned or broken, the compression starts to go low in one of the cylinders. 

Along with that, a blown head gasket is a cause of low compression too. At the top of the engine, the head gasket stays connected to the cylinder head. 

If your head gasket gets blown, you’ll notice there’s a gap between the head and the cylinder cap. That gap leads to the gas flowing out – causing a low compression in one of the cylinders. 

Another cause of a failing head gasket is overheating. Your boat’s outboard motor operates through conditions like immense heat. The engine of your boat can overheat if this heat is constant– resulting in a blown head gasket. 

Overheating can turn things upside down for your engine. With too much heat, the engine block and cylinder tend to expand a lot-causing a head gasket failure. And a head gasket failure is a reason why the compression gets low in the cylinders. 

2. Leaking Head Gasket 

A drastic change in the engine power can be noticed when the compression lowers in any cylinder. This scenario is a result of a leaking gasket head. 

And a leaking head gasket is a sign that points out problems in the engine. 

Sometimes, you can see white flames coming out of the motor. This symptom is a clear sign of a bad head gasket. 

3. Worn out Head Gasket 

However, it’s not just overheating and leakage that can cause a low compression for a failing head gasket. There are also cases where the head gasket wears out. 

And when a head gasket wears out, it automatically causes poor compression in the cylinder. 

4. Carbon-Clogged Piston Rings 

The clogging of piston rings with carbon is another common cause of a low compression reading in your engine. Plus, the outboard motor or your boat goes through a constant combustion cycle. This cycle leaves behind a lot of carbon residue. 

When carbon continues to pile up, the piston rings get stuck. For this situation, the piston rings fail to make a good deal against the cylinder walls. So, if this condition keeps going on, the outboard will go through a lot of damage from the carbon residue. 

5. Broken or Worn Piston Rings

Pistons have multiple rings to keep the wall of the cylinder sealed. The top ring works mostly in sealing the cylinder, whereas the secondary catches the missed parts. There’s a third ring that is known as an oil control ring. 

This ring removes oil splashes off the cylinder wall while the engine keeps running. 

These rings break down and wear out over time. And a worn-out or broken piston ring is a sign of internal damage to the engine that causes a low compression in cylinders. 

Besides, piston rings can quickly get overheated. When too much heat falls upon the engine of your boat, holes start to form in the pistons. At that time, rings can no longer seal the pistons, which causes a gas leak. 

And when gas keeps leaking out, the compression keeps getting lower too. 

Usually, pistons are strong enough to withstand damaged combustion. But they burst out because of overheating– causing low compression.  

6. Valve Issues 

Every outboard engine has an exhaust and intake valve. The exhaust valve lets out gas and helps in combustion, whereas the intake valve allows in fuel and air. 

They actively take part in the combustion process of the cylinder. Fuel and air enter into the intake valve, and the formed gas gets out through the exhaust valve. 

These valves tend to wear out and leak out vigorously when exposed to too much heat. And Overheating causes the metal ring to loosen up and fall from the cylinder head. This scenario allows air to escape to the valve port and cause leakage. 

When there’s too much leakage of the gas, the compression ends up lowering from the cylinders. 

7. Broken Camshaft 

In an outboard, every valve has a camshaft. When a camshaft breaks, it causes failures in the crankshaft. In that case, it becomes incapable of pushing the pistons accordingly. 

An absence of pressure is seen in cases like this, which in turn causes low engine compression. 

This case is rare and has lower chances of happening in comparison to the other causes. But that doesn’t mean, it can’t be the reason.

8. Worn-down Camshaft Lobe 

Every camshaft has a lobe to function in the outboard motor. When the valve doesn’t seem to open up, it means the camshaft lobes become worn out. And that happens when the valve doesn’t open. 

At that time, the cylinder is unable to let air in or let the air out through the exhaust gas– causing compression issues. 

The worn-out camshaft lobe causes it to open and get stuck in an enclosed position. And by the time the lobe wears out, the compression gradually starts to get low than its actual reading. 

9. Worn Timing Belt 

Timing belts might sound like an irrelevant tool to the compression, but it’s not. They’re kept engaged in the engine for controlling the piston by latching the camshaft and crankshaft.

When there’s an issue with the piston, that means there’s an issue with the compression too. 

So, the camshaft won’t be able to function when the timing belt breaks or wears out. This situation causes the exhaust valve not to close off while the intake valve opens. 

As a result, the gases will get stuck and pile up in the combustion chamber. And this scenario is a cause why the compression lowers in the cylinders. 

10. Cracked Cylinder Wall 

Cylinder wall, also known as Cylinder Block, tends to crack open when exposed to too much heat. And when the cylinder wall cracks, the compression gets affected– lowering the reading. 

But if you want to be sure whether the cause of the low compression is a cracked cylinder wall, then there’s away. All you have to do is turn on the engine and keep the radiator open. And then wait to see if any air bubbles pop out.

If you detect bubbles, then keep in mind that these bubbles are gases that come out of the cracks of the combustion chamber and fly to the cooling system. Thus, that’s when the compression lowers in the cylinders.  

Can you Fix One Cylinder Outboard With Low Compression? 

It’s a common question among most people who face issues with low compression. But you shouldn’t worry if you have the same question too. That’s because the answer to this question is, Yes! 

Depending on the size of the problem and how severe it is, you can do certain things to bring your compression up and fix it. 

But before doing any fixes, you have to make sure that you’re ready to bear the costs of the needed parts and a mechanic. As long as you’re okay with that, it’ll be effortless for you to fix the low compression of an outboard. 

No matter the issue, you always have to do a compression test to check if the compression is low or not. If you don’t do this test, you won’t figure out the exact problem and end up causing further damage to your vehicle. 

Sometimes, just by doing a compression test, you can come across the problem immediately.  

How to Test Low Compression in One Cylinder Outboard

Well, testing the compression in one cylinder outboard takes a heck of a time. But it’s worth the time you’re going to invest. Let’s check out.

For Gasket Issues 

First, measure the compression of the cylinders using a compression tester. Keep in mind to specifically check for gasket readings and see whether it’s different than normal. 

If it shows to be problematic, then shut your engines off and stay away from releasing pressure. 

Take your vehicle to the closest technician and use original replacement gaskets. 

For Carbon Build-Up

When there’s a bar on the build-up and causes a valve issue, there are products that can help you out with this problem. These products can help by being added to the fuel, which cleans out the carbon. Using these products can flush out the carbon and stabilize the compression. 

For Piston Ring Issues

Like gaskets, pistons are a critical component to the Outboard. Hence, you must do a compression test and detect whether it’s a pistons ring issue or not. If you find it to be low, then replace the piston rings to avoid further damage.

For Valve issues

To detect valve issues like leaky valves, run the motor. Instead of the spark plug, use a compression tester and run the engine to detect the valve issue. Look for a gas leak on the exhaust valve and inlet manifold. 

If you hear a sound, that would mean there’s a valve issue. As soon as you find your valves to be damaged, remove the cylinder head and do a valve job. 

For Camshaft Issues

By removing the valve cover flipping the engine, check out the valves thoroughly. Keep an eye on the valve movement that occurs. When you notice an issue, it’s sure there’s a faulty camshaft. At that time, simply replace the camshaft. 

As soon as you read the above facts, you can learn about the one-cylinder outboard and the issues it can face.

But the fun part is whatever the problem may be, you can always fix the compression rate by just replacing the faulty parts. 

How Much Does it Cost to Fix Low Compression in One Cylinder? 

When compression starts to get low in one cylinder, let alone two or three cylinders, there’s a possibility your engine might have damaged parts inside your boat. And every vehicle comes with maintenance and continuation. 

Thus, always keep in mind that when it comes to repairing any of your vehicles, you should always be ready to bear costs for the better health of the vehicle you own. And repairing vehicles requires you to take it to the technician. 

So, you have to be prepared to bear the expenses as they might vary from one technician to another. 

In-vehicle repairs, most of the work goes on replacing the damaged part. And the overall fee comes with the cost of the replacement. 

A professional mechanic will charge you $100 to $200 for fixing low compression in one cylinder. However, sometimes while repairing there are other replacements that might need fixing. At that time, the cost is going to vary. 

For example- repair of the damaged head gasket will cost you from $1000- $2000. Replacing the piston rings will cost you about $2000- $4000. But the most costly repair is the replacement of the whole cylinder, which is $8000. 

Fortunately, you don’t have to worry about all that expenses as you will need only $100- $200 to fix one cylinder with low compression. 

How Do You Fix Low Compression in One Cylinder? 

If you went through the section where we’ve explained the causes of low compression in one cylinder, you know there’re many internal parts that can cause the problems. But whatever is causing the problem, you must do a compression test first.

The compression test doesn’t take that much time and will be done within 45 minutes. When you’re checking for low compression, make sure to also check for where the issue is coming from. 

Like – gasket, pistons, valves, and cylinder. And problems in any of these parts can be the reason for low compression in one cylinder. 

So, to fix the issue from the root, you have to know the cause first. And you’ll find loads of information about your boat in the manufacturer’s details. 

But there’re some things you can do to fix low compression. They are: 

1. Confirm if the compression is low. Before diving into the thought of having an outboard with low compression, you have to make sure of it first. To know whether low compression is the problem or not, always use a compression tester.

2. Use the compression tester to detect whether it’s the compression or another part of the engine that’s causing the problem. 

3. Figure out the key cause of this issue. Go through the causes and detect what the problem is. 

4. Replace the damaged parts. Most cases involve- damaged valves or piston rings and gaskets. When there’s an issue with these parts, it’s a necessity to replace them and install a new part. However, if it’s the timing belt causing the problem, pour some oil into the cylinder. Doing will fix the issue. 

5. To check if the repair worked, do a test drive. Testing the replacement is the best way of knowing whether the repair worked or not. Always observe the boat’s performance level. When there’s no fickle or stalling, you’ll know the repair worked. 

Fixing the outboard all by yourself is a hard task to pull of. But that doesn’t mean you can’t do minor repairs. Sometimes you might feel like the repair is out of your reach and capabilities. 

At times like that, you can always reach out to a professional and get your boat repaired. 

Is it Possible to Drive With No Compression in One Cylinder? 

Cylinders play a big role when it comes down to the smooth functioning of your boat’s engine. The power that operates the motor, in produced in the cylinder. It’s also the place where all the fuel is transformed into mechanical energy. 

Mechanical energy is the food of every vehicle. And the process of creating mechanical energy follows a process named compression. To ignite the engine, compression is the first step to generate power. 

Inside the motor engine, there’s a cylinder with pistons inside it. These pistons move forward and backward. 

Pistons work by compressing fuel and air together to get the correct amount of mechanical energy required to run the engine. And having a low compression spoils the whole function of the engine. 

However, a boat has multiple cylinders. Sometimes, it’s a two-stroke or four-stroke engine. At that time, when there’s only one cylinder that’s low in compression, you’ll face rough movements. But, the boat will not stop entirely. 

Your boat will still run but not as fast. It will stop from time to time but start again. These rough kinds of movements will the outcome when one of your cylinders is low in compression. 

So, yes, it is still possible to drive a boat. But it’s advised not to do so as it might cause further damage to your engine. 


Problems in the core parts of a boat can quickly cause damage to the entire vehicle, especially when it’s the outboard motor of your boat that is facing these issues. And that includes the problem like low compression. 

But that doesn’t mean you can’t fix low compression in one cylinder outboard, as we’ve already mentioned the tips and tricks on how you can detect and fix it. So, don’t worry when you face this issue next time as you now know what to do. 

Good luck!

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