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What Should I do If My Marine Engine Misfires or Has Rough Idle?


An unexpected change in the performance of your boat motor can put a damper on a day of fun on the water or even prevent you from leaving the dock. A misfire will cause the engine to momentarily stumble or lose rpms and then regain its normal engine speed. The misfire will usually reappear, either under specific operating conditions or randomly. A misfire may occur when your engine is idling, causing a rough or uneven idle.


Misfires and rough idling are difficult problems to isolate, since they can be caused by many system malfunctions. 

  • Ignition system components

    -Spark plugs

    -Plug wires

    -Ignition coils

    -Ignition timing

  • Lean air/fuel mixture
  • Mechanical issues within the engine
  • Worn piston rings, valves, cylinder walls or worn cam lobes
  • Vacuum leaks in the intake system that can cause misfiring

A misfire usually starts out as being merely a nuisance but if the causes aren’t identified and corrected, it may result in your engine not starting or stopping altogether when you least expect it. 



Sluggish or rough acceleration

When the engine misfires, you may have trouble getting your boat up to speed. Misfires often happen when your vehicle is under load while accelerating. This can result in slow or sluggish acceleration or a jerking motion while pressing down on the accelerator.


Rough idle

A rough or uneven idle can be an indicator of an engine that is misfiring. The air/fuel mixture is disrupted which can cause the engine to jump up and down. 


Changes in engine sound

If your marine engine is misfiring, you’ll likely hear it. Does it sound different than normal? Unusual sounds can be a clue that there is something going on with your engine. 



If you’re experiencing a misfiring engine, you’re likely going to need to enlist the services of your mechanic. Before you take your boat or engine to the shop, gather as much information as possible to assist your mechanic in diagnosing the problem. 


While you’re out on the water, pay close attention to how your engine is running. When a misfire occurs, take note of the circumstances such as whether the engine is cold or has warmed up; the speed at which it occurs (low speed or high speed); the frequency it occurs; and if it occurs only when accelerating or at a steady speed. Keep a log of the problems, this information will help your mechanic pinpoint the source of the problem.

Armed with this information, your mechanic will have your boat engine hitting on all cylinders and running smooth in no time.

Learn more about marine spark plugs & marine batteries, find your part, or find where to buy your part today.

The content contained in this article is for informational purposes only and should not be used in lieu of seeking professional advice from a certified technician or mechanic. We encourage you to consult with a certified technician or mechanic if you have specific questions or concerns relating to any of the topics covered herein. Under no circumstances will we be liable for any loss or damage caused by your reliance on any content.

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