If you are having trouble with your boat motor stalling, it may be losing compression. While any kind of mechanical problem can be frustrating, this goes doubly for something on your boat which is supposed to be this awesome piece of adventure and technology.
There are a lot of reasons your boat motors lose good compression and, thus, lose power. We’re going to dive in to these compression issues, give you some tips about how to check your boat engine’s compression and vital components, and ask why do boat motors lose compression?
Table of Contents
- Why Do Boat Motors Lose Compression?
- Checking Your Boat Motor Compression
- The Reasons Your Boat Motor is Losing Compression
- Carbon Clogging the Piston Ring Grooves
- Engine Could be Worn Out
- Internal Damage
Why Do Boat Motors Lose Compression?
Here are some of the major reasons that boat motors lose compression:
- Carbon clogging the piston-ring grooves
- The engine could be worn out
- Internal damage (a broken piston ring, for example)
It is also possible that when you do a compression test that your readings will say that nothing is wrong, which you should document as well. There may be a more sporadic problem happening inside your boat engine motor.
Before we discuss these results, we are going to talk about how to check the compression on your boat motor so that you understand what you need to do. From there, we’ll break down these major reasons that boats lose compression to help you understand what your situation means for you and how to fix outboard low compression issues.
Checking Your Boat Motor Compression
A compression check is a great thing to do if you are curious about the health of your boat’s engine. If you have never performed a compression check before, you may be asking why you would want to do a test like this or how it is performed.
The compression that an engine has is its ability to compress the gas and air mixture that is required to make the engine operate.
Outboard Low Compression Symptoms
An outboard gas engine actually needs five things to operate correctly. It needs air, fuel, a spark, and compression, and it needs to be able to output exhaust. It needs all of these things in very specific amounts and at very specific times. If you’ve ever thought that your engine is probably a fairly complex machine; you would be correct.
To make all of this happen; the piston in your engine travels up and down in the cylinder to compress the air and fuel mixture at the top of the cylinder at the right time.
If there is a failure with your piston, it is not going to result in the correct compression of the air and fuel that is being mixed. If that mixture isn’t compressed correctly; the spark will not ignite in the combustion chamber, and your engine will probably not run at all; although if it is running, you’ll experience a severe loss of power and frequent misfires.
Also, if you have rings that are not doing their job when it comes to the piston and the cylinder wall, they will not be transferring heat correctly in and out of the system, which is going to lead to more potential damage inside of the cylinder due to overheating. It can be somewhat amazing how all of these systems have to work together so perfectly to make everything work the way it should.
One of the best ways to check your engine is by checking your engine’s cranking compression because it will reveal a lot of information about your engine.
This compression check that is performed is going to tell you if your piston rings are working properly and that your cylinders are in good shape as well. This test looks at how much pressure gets built from the piston moving in the cylinder, and it is recorded in pounds per square inch.
While this is a simple test for many mechanics and technicians to perform, you probably do not need a ton of experience to perform a diagnostic test that is rather simple.
However, if you are not performing this test, this information can still be valuable because you will have a total understanding of how everything works.
If you are not going to do a compression check, you or a technician may want to perform some other tests that can help measure metrics inside of your engine as well, including a leak-down test which measures how much air is leaking from the cylinder.
If your pistons and cylinders have major problems, these other kinds of tests can help you measure whether your repairs are going to be more expensive or not.
In theory, it should be fairly easy to perform a compression test on a motor; however, there are many different types of motors today that can make this test a lot more difficult.
If you are thinking about performing any kind of maintenance on your motor, you should be sure to check with your manufacturer’s instructions and recommendations as well as any information for your specific model that may not fit into these general guidelines that are offered here.
Your engine could be different depending on whether it is a two-stroke or four-stroke engine. There are additional features that can cause differences as well like a fuel injection motor or an engine that works with a computer setup.
If you are dealing with a motor that you think is going to be more complex; you should probably leave the job up to some professionals. As technology continues to get more complicated, it will be harder to do this service on your own.
If you are destined to complete this job on your own; you’ll need to pick up a compression gauge. You can find one of these at most auto parts stores that you could go to, and anyone there should be able to help you pick one up; especially if you are just going to use it every once in a while.
The one thing you should be aware of is that you may need an adapter to fit where the holes for your spark plugs are. If you’re talking to a Customer Service representative, just make sure they know what you are trying to do, and they should be able to help you out.
While some people will tell you that you should check your compression on a cold engine; it is probably wiser to do so on a warm engine. When you are checking a warm engine; you get a reading that is much more accurate to your boat when it is actually running and in the water.
If you’re trying to check your compression while you’re outside the actual body of water that you operate on, you’ll simply need to check with the manufacturer’s instructions to see if you need anything extra. You may need a bit extra setup, so be warned if you are storing your boat out of the water and want to do this check.
If you do want to check your engine when running warm, you simply need to start it up and let it run for a while so you can get an accurate reading of what your engine performs like during the boat’s actual use.
To start, you are going to want to check your engine’s spark plugs by removing them. When looking at the electrodes, you should see that they have the correct amount of space in them.
They should be anywhere from a tan color to a brown color if you have been running the engine consistently. If they look like they are more white than tan, the engine isn’t getting enough fuel into the system, which may be a major problem that definitely needs your immediate attention.
Prevent the Ignition From Firing
Step number one when checking the compression in your motor is to be cautious of the spinning propeller on your boat. You need anything possible to ensure that the ignition doesn’t fire because the propeller will start up and be extremely dangerous if you start the engine up.
Usually, this is going to be easy to accomplish by pulling an engine kill switch. However, in some models, you may have to disconnect the battery, and if so, you would need to reconnect it for an electric start.
Again, you’ll need to remove the spark plugs as well and make sure that you have a labeling system in place so that you know exactly where they need to go back.
If you have an ignition when the plugs aren’t properly grounded, you are probably going to be damaging the ignition. A spark occurring could also ignite vapor that could be left over in the empty cylinder, which would also be a bad situation.
If your engine is one that requires the kill switch to be “off” to crank, you may have to use a spark checker to proceed. These are also available at a lot of different auto parts stores.
Move the Throttle Forward
If you have an electric start motor, you will probably not be able to do to this and crank the engine without a bypass button.
If this sounds like it may be difficult, check the owner’s manual. Also, your engine may be good without the throttle forward to facilitate that air so you could be in the clear in that instance.
Screw the Compression Gauge into First Cylinder
When doing this, you should be certain that the plug leads are not in the way of any moving parts. Obviously, failing to do that is going to lead to damage when all of the moving parts start into action.
Crank the Engine
You’ll need to use the starter or the pull cord through around five flywheel revolutions. You’ll know you’ve done this enough because the compression gauge needle is going to stop going higher.
Your engine won’t be starting with the plugs removed, but there will be fuel in the cylinder, so be careful with any smoking or sparks that could ignite those gases. If your engine is one that is fuel-injected, this could be very dangerous vapor that gets into the cylinders unless your engine has a way to prevent that.
At this point, you would need to check the reading for each specific cylinder; repeat the process for each different cylinder inside of the motor and make sure to record the results.
The Reasons Your Boat Motor is Losing Compression
If you look at the information that your engine manufacturer provides, you will not see a specific cranking compression standard. The reason that they do not provide this kind of information is that there are too many variables that healthy boat motors operate with.
Generally, the best practice is that your readings should all be within 10 PSI of each other. This basically means that all your cylinders are working appropriately because they are all working the same. It would be very rare for your compression to be messed up evenly across all of the cylinders.
However, many consumers want a little bit more information when it comes to these readings. They want to know if their readings are in line with other people who use this motor to make sure that their performance is acceptable. You could definitely speak with the dealer you bought from to see if your readings make sense for your type of motor.
Generally speaking, any reading below 90 PSI will indicate some kind of a problem. Most modern two-stroke outboard engines will produce readings somewhere between 90 and 110 PSI. Even this range has a lot of different variables that could change your readings based on how new your engine is and what kind of model it is.
If you have an engine that is much older or you have a high-performance engine, you will see a PSI reading even higher than the numbers above. Your listing will probably be between 120 and 140 PSI. You should keep in mind that there is a lot of variances here, so for example, a reading of 119 PSI may not be a huge issue. Most of these readings are relative to each other.
If you think your compression readings are low or they have a lot of variance between the different cylinders, here are some of the most common reasons why you would be getting readings like that.
Carbon Clogging the Piston Ring Grooves
This is one of the most common causes that will have owners seeing low compression. When your engine goes through the combustion cycle, it leaves a lot of carbon residue behind.
If this continues to happen over time, it makes the piston rings get stuck, and they can’t form a good seal against the cylinder walls. If this condition continues to happen over time, you are going to get a lot of engine damage from this leftover carbon.
One thing that you can do to prevent this issue is to add a product that prevents this to your fuel, or they also make products in spray form as well. If the carbon residue has hardened or is built to an extreme, you may need to completely disassemble your engine and get it cleaned and rebuilt.
Engine Could be Worn Out
This is a problem that happens with older motors. Sometimes the cylinders will simply be worn from their original shape, or they’re simply tapered after years of use. When this is the issue that you are dealing with, you will usually see all of your cylinder readings showing up low, as opposed to just one cylinder.
However, as opposed to simple wear and tear, you could also be dealing with damage inside the engine. This could be something like a broken piston or a broken piston ring, a bad valve seating, or just any piece of the engine that is completely busted that should be doing a job in the combustion process.
If you have significant internal damage, it is highly likely that you will see one reading much lower than the rest, potentially even at 0. If you have an engine where a cylinder is not firing at all, you are obviously going to have bad performance or possibly an engine that won’t run at all.
If you are in a situation where you’re getting very low or crazy readings, you may have to remove the cylinder head to get in and inspect everything going on. At that point, you would probably want your mechanic working on this problem if you are still considering it yourself.
It’s possible that you think you have some kind of performance issue with your engine providing normal readings. If that is the case, you’ll want to record the results in case you are looking to sell your motor in the future.
This is a basic test that you can do on your motor. However, there are a lot of different variables and specifics that can’t be covered with basic guidelines. If you do not follow the appropriate steps provided by your manufacturer, you could be entering a dangerous situation resulting in damage or injury fairly quickly.