What Causes a Boat Motor to Lose Power



The question you can’t get out of your head after a frustrating day on the water—what causes a boat motor to lose power? We’re going to set aside the possibility that you’re simply out of gas and explore the other likely options. This said, did you check your gas, just to be sure?

Just like the engine in your car, truck, or SUV, your boat’s engine needs three things to operate properly:

  • Air

  • Fuel

  • Spark to ignite the air/fuel mixture

If air or fuel is out of balance, your engine will run sporadically or maybe not at all—a weak or improperly timed spark will have the same result.


Why is my outboard bogging down or losing power altogether? Here are the top 9 sources of the problem.


In-Line Fuel Filter

We always recommend carrying a spare in-line fuel filter. If your engine is losing power, first replace the in-line fuel filter. If you’re not carrying a spare, remove the filter, clean the filter element of debris, and drain any accumulated water.


Spark Plugs and Spark Plug Wires

Spark plugs can be fouled by fuel, oil, and/or carbon.
Spark plugs can be fouled by fuel, oil, and/or carbon.

A spark plug fouled by fuel, carbon, dirt, or oil may be unable to produce a spark strong enough to ignite the air/fuel mixture and start your engine.

Inspect each spark plug and also inspect the wires. If your plugs are fouled, use a rag to remove the build-up as best you can. If your spark plugs have been neglected, you might need to use a knife or similar object to remove larger pieces of build-up. However, be careful not to damage the spark plug or accidentally change your spark plug gap.

If your spark plugs were significantly fouled, replaced them as soon as is possible.

Then, inspect your spark plug wires, checking for signs of aging such as cracking or brittleness. Next, inspect your terminals and connection points. If there are signs of corrosion, damage, or breakage, you will need to replace some of your terminals. However, make sure you use the proper crimper when installing terminals, or you might create a weak point in your electrical system—creating a new problem.


Untreated Fuel

Thankfully, there is a very wide range of fuel stabilizers on the market.
Thankfully, there is a very wide range of fuel stabilizers on the market.

It’s possible to buy bad fuel, but it’s more likely that your fuel went bad sitting in your boat. If you left your tank nearly empty for long periods, condensation could build, introducing water to the gasoline. Your gas might also be bad if you did not properly treat it before storage. If this is the case, add a fuel stabilizer and make sure to run your engine long enough to get the treated gas into the engine as well.


If this is the first time you are running your engine after storage, especially if you did not add fuel treatment designed for storage, you might need fresh gas. Fuel can go stale in as few as 30 days, especially fuel mixed with ethanol because ethanol attracts moisture over time, ultimately diluting the gas.


How does ethanol-gasoline cause problems in a marine engine? Ethanol prefers to bond with water rather than gasoline. Through the natural process of condensation, your boat operating in a water environment, and accidental introduction of water into the fuel system, water finds its way into the fuel tank.


Water is heavier than gasoline. If both water and ethanol gas is in the fuel tank, water will sit on the bottom of the tank – your fuel pump is also at the bottom of the tank. This is a problem, but the problem is about to compound. If there is enough water is in the fuel tank, ethanol will leave the gasoline and bond with the water, potentially doubling the volume of the water in your tank. You can end up with water enriched with ethanol rather than ethanol-enriched gasoline.


Fuel Pump Blocked or Going Bad

Is the problem with your fuel pump? If your fuel pump is whining or squealing, it is typically one of two reasons. First, the pump might be starving for fuel. Fuel lubricates fuel pumps; if your fuel filter is blocked or clogged, it may be prohibiting gas from getting to the fuel pump. In this case, the problem is likely a blocked fuel filter, not necessarily a bad pump.


The second reason a fuel pump might whine or squeal is that it is in the initial phase of failing. If you have changed your fuel filter and the problem persists, your fuel pump is likely going bad and you should replace it right away.


Airflow Sensor Gone Bad

If your marine engine is fuel injected, a dirty or damaged airflow sensor can reduce your engine’s performance. If other attempts are not addressing the problem, consider cleaning or replacing the airflow sensor.


Fuel Line Damage

What causes a boat motor to lose power? Could be fuel line damage.
What causes a boat motor to lose power? Could be fuel line damage.

Is your fuel line cracked, deteriorated, or damaged? The fuel line is definitely at risk of deterioration and can contribute to stalling and poor engine performance.


Low Compression

Worn piston rings, cylinders, and leaking valves can lead to low compression in your boat engine. If your boat’s engine lacks proper compression, the engine will struggle to finish the combustion process, and performance will suffer.


Dirty or Worn Carburetor

If your carburetor is dirty, it can’t effectively regulate the air and fuel mixture needed for your engine to run correctly. This can lead to stalling. Also, if your boat’s engine is equipped with a shift linkage, it may be worn and contributing to or causing stalling problems.


Idle Air Control Valve

The idle air control valve regulates your engine’s idle speed, and an idle air control valve that is not working properly might cause the engine to stall.


We’ve covered nine of the top reasons to answer the question – what causes a boat motor to lose power. Hopefully, these top reasons covering common boat engine problems will help you identify the problem and enjoy your boat’s engine performing at its peak.

The key to long engine life and many years of outstanding engine performance is diligent and regular maintenance.

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