3 Reasons Why Your Boat Motor Have Lost Compression (Solved)

Like any other machine, boats might have a few issues here and there that might require fixing. While these problems range from structural to mechanical problems, they might reduce the efficiency and comfort of using your boat. Losing boat motor compression is one of the most popular mechanical problems you might encounter from time to time while using this excellent piece of tech.

However, the fun doesn’t end with your boat motor losing its compression. All you need is the right diagnosis of why the boat motor has lost compression and fixing it. Here, you will acquire the basic knowledge you need to check your boat engine’s compression and give the three reasons why your boat’s motor might have lost compression.

What Reasons Explain the Loss of Compression by Your Boat’s Motor

Your boat’s engine manufacturer in most cases will not provide information about the specific cranking compression standard. This follows the reason that there exists a myriad of variables that a healthy boat motor operates with. You, therefore, need to seek ways of how you can check your boat motor’s compression. The optimal and best practice is that boat motor compression readings be within 10 PSI of each other.

Basically, this implies that all your cylinders are working optimally since they are all functioning the same compared to each other relatively. Rarely, will you have compression messed up evenly across all cylinders lest the whole motor stalls.

Many boat users wish to have some little information about these readings. This is to compare their readings with those of other users to check if they are in line in a quest to seek validation of performance. However, speaking to the dealer who sold you the boat is recommended to have the dealer tell you if your readings make any sense for your boat’s motor type.

In most cases, readings below 90 PSI in a significant way to signify some kind of problem. Nearly all modern two-stroke outboard engines produce readings between 90 and 110 PSI. Nevertheless, this range also has a lot of different variables that could change your readings based on the engine model or how new or old it is.

For older or high-performance engines, a PSI reading even higher than the above numbers are expected. Notably, your listing will probably range between 120 and 140 PSI. Therefore, keep in mind that there remains lots of variance in this case. For example, there might be no big issues for a reading of about 119 PSI. Generally, most of these readings are relative to each other.

However, if you presume that your boat motor compression readings are low or they have a lot of variance between their cylinders, here are Three Reasons Why Your Boat’s Motor Might Have Lost Compression.

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A Worn-Out Engine

Three Reasons Why Your Boat’s Motor Might Have Lost Compression

In most cases, a worn-out engine might be the cause of your boat engine’s loss of compression. This happens mostly for boats with older motors or boats that are generally old. The engine’s cylinders might be worn out from their originals shape or tampered with due to wear and tear. You will see all the readings of the engine’s cylinder showing up low in such a case. That is as opposed to a single cylinder.

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Carbon Clogging the Piston Ring Grooves

One of the reason why your boat’s motor might have lost compression is the carbon build-up that results from the combustion cycle of the boat’s motor. Continuous carbon build-up from the residue over time will in most cases make the piston rings get stuck, making them offer a poor seal against the cylinder walls. If this condition persists lots of engine damage can be expected from the excess carbon build-up.

Three Reasons Why Your Boat’s Motor Might Have Lost Compression

To prevent this, you would need to add a product that prevents this from your engine’s fuel. Otherwise, you may seek some products that come in spray form for the same reason. However, if the carbon residue has hardened or built up to an extreme, you might need to consider disassembling the whole engine and have it cleaned up or rather, rebuilt. While this process seems tough to do on your own, seeking some professional mechanic on boat motors might be your solution in such a case.

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Internal Breakdown, For Example, a Broken Piston

Other reason why your boat’s motor might have lost compression are wear and tear issues, you might be battling with more severe problems. Internal breakdown issues might have plagued your boat motor. Some common internal breakdown issues include a bad valve setting, broken piston(s), broken piston ring(s), or any broken piece of the motor. Breakdown of any piece that is tasked in the process of combustion means your motor would even stall.

Three Reasons Why Your Boat’s Motor Might Have Lost Compression

With internal breakdown issues, you probably see one reading much lower compared to the rest, potentially even at 0. Additionally, an engine where the cylinder fires, not you expect to have bad performance or even an engine that won’t run by whatever means. In a situation where you obtain low readings, you might opt to remove the cylinder head.

This is to help you have a better look at what might be wrong with the engine as you inspect everything. Seeking some mechanical help from a professional at this point is recommended unless you are one or know how to go about it.

Others are times you presume to have some performance issues with your boat motor even when the engine provides normal readings. In such a situation, consider taking the readings if you might be looking forward to selling your motor in some near future.

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Final Thought

The above-stated Three Reasons Why Your Boat’s Motor Might Have Lost Compression will be automatically the number one thing to check for a boat whose motor has lost compression. Notably, this offers some DIY basic tests for your boat’s motor. However, there are lots of different variables as well as specifics that can’t be covered within the basic guidelines.

It is highly advisable that you seek some professional help from a mechanic to avoid damaging your motor if you lack basic skills with mechanical objects. A manufacturer guide might help you avoid some of the issues while dealing with the situation, hence avoiding further damage. We hope that you have found this article beneficial and that we have answered the question; Three Reasons Why Your Boat’s Motor Might Have Lost Compression


Hi, I’m John who owns multiple different boats and i love all kinds of water activities. From boating to fishing, from scuba diving to windsurfing, from water tubing to snow tubing.

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