How to Test the Compression on an Outboard Boat Motor

How to test the Compression on an Outboard Boat Motor

Compression is the most important aspect of an outboard boat motor. If you lose compression- You lose the motor!

Maybe you are in the process of buying a used boat or suspect your current outboard motor isn’t running properly due to a loss of compression;

what you will need to do is conduct an outboard compression test.

What is a Compression Test?

A compression test is a test done on any type of combustion engine that is designed simply to determine if all of your:

  • Piston rings
  • Valves
  • And Gaskets

are providing a proper seal in order to achieve the required compression for combustion.

If you have a cylinder that lacks the required compression, then the pistons do not move up and down properly in order to compress the air and fuel mixture that eventually combusts.

The Parts You Will Need To Perform A Compression Test

In order to perform a compression test on an outboard boat motor, you will need the following:

How to Perform a Compression Test on an Outboard Motor

Now that you understand what an outboard compression test is, it’s time to start testing.

Listed below are the steps necessary to test the compression of an outboard motor:

  1. Remove the cowling of the motor. This will give you access to the cylinders.
  2. Remove any other covers that might be over the ignition coils or spark plugs
  3. If your engine has ignition coils, remove the bolts that hold in the coils and then remove the coils.
  4. Remove the spark plug wires from atop the spark plugs (mark them so you can put them back on the proper cylinder later)
  5. Remove the Spark Plugs (This is a great time to replace them with new spark plugs!)
  6. Place the compression gauge on the spark plug hole of one of the cylinders.
  7. Turn the key or push the button to start the ignition of the engine you are testing (It will not fire as you have removed all of the plugs)
  8. Once the compression readings stop going up, record that number and move on to the next spark plug hole and repeat steps 6 and 7
  9. Compare the readings from each cylinder and make a note of any that are significantly lower than the others.
  10. If compression is low in any of the cylinders, you will need to have the engine repaired or replaced.

Proper Compression for a 2-Stroke Outboard Motor

proper compression for a 2 stroke outboard motor

2 Stroke Boat Motors should have a compression reading between 110-130 PSI.

It is possible that you could get a lower compression number on a cylinder that’s still running perfectly fine, the problem is that it isn’t.

That low number is a sign that this cylinder is on its way out sooner rather than later.

Proper Compression for a 4-Stroke Outboard Motor

proper compression for a 4 stroke outboard motor

A 4 stroke boat motor should have compression results between 180-210 PSI.

Unlike with 2 stroke motors, you might get a lower reading on a 4-stroke but that isn’t an indication of a bad cylinder.

This may just mean that you have to open up the throttle plates on that specific engine to let a little more air in, and that should bump up the compression numbers.

When Should You Do A Compression Test On A Boat Engine

There are two times when you would be doing a compression test:

When you are buying a used boat:

When you are buying a used boat, compression testing essentially gives you a breakdown of where the engine stands and how it was used.


Even a passing compression test that results in numbers on the lower end of passing likely indicates a motor (or a cylinder) on its way out.

When you believe your motor isn’t running properly:

After using your boat for several years, you will know what a normal outboard motor sounds and runs like, and you will know when something is off.

What Signs Indicate A Compression Test Should Be Done On My Boat Engine?

There are several indicators that you should watch out for which suggest there could be problems with your boat engine’s compression:

  • Hard starting – If your engine takes more than two or three cranks to start up, this could be indicative of low compression in one or more cylinders;
  • Rough running – Low power output from the engine at idle or under load could also signify lower-than-normal compression;
  • Poor fuel economy – This usually happens if the air/fuel mixture in the cylinder isn’t optimal due to lower pressure levels within the chamber.

Can I Perform A Compression Test On My Boat Engine Without A Professional Mechanic?

The answer is yes! To perform a compression test, all you need is a compression gauge and a few other supplies like oil, rags, and gloves.

The process isn’t overly complicated and doesn’t require you to hire a professional mechanic.


In conclusion, it is important to use the right type of compression tester and understand the risks associated with performing a compression test on an outboard motor.

Compression tests should be done regularly to ensure proper engine performance and safety for boat operators.

If any other signs indicate a potential problem in your boat’s engine, you may want to consider having a professional mechanic perform a compression test as well.

Although it is possible to do a compression test without assistance from a professional technician, it is best practice to have one look over the results of your testing if you are not experienced with working on engines yourself.

Doing so can help identify underlying problems that could cause damage or failure if not corrected immediately.

Taking the time to diagnose and address issues properly will save money in the long run by ensuring the optimal operation and reliability of your outboard boat motor.


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Steve Momot

Steve Momot

Steve is an accomplished professional photographer and marketer who specializes in the Fishing, Yacht, and Boating industry. With a strong presence as an influencer and marketing expert in the Marine Industry, he has made a significant impact in the field. Additionally, Steve is the original creator and co-founder of Sportfishtrader. Prior to his career as a marine photographer, he gained extensive experience as a licensed boat and car dealer in South Florida.