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What causes a marine engine to have mIsfire/rough idle?

Misfire/Rough Idle

Boat engines have the same operating principles as do land vehicle engines, but having trouble with your boat engine can leave you and your family in a precarious predicament. If your car, truck, or SUV doesn’t run up to snuff, you can pull off to the side of the road and call for towing assistance. In a boat with a malfunctioning engine, even if you stop and drop anchor to wait for help, you’re at the mercy of the weather. As any experienced boater knows, storms can crop up over open water very quickly and can pack a powerful punch.

Chances are that an engine misfire or an engine with a rough idle may not leave you stranded, but it will limit your speed and maneuverability to outrun foul weather, high winds, and strong tides. Should you notice these issues before leaving your dock, a wise decision would be to stay tied up at the dock until the problem can be sorted out. 

What to Look For

The first thing to do is to analyze the capabilities of your sick engine. If it’s a misfire, try different engine speeds and note if the misfire is consistent at all rpm, or if it varies or disappears altogether at certain speeds. Also note if it happens when the engine is cold, warm, or both. Having the answers to these questions will help you focus on the precise issue when it comes to asking for assistance.

Remove the engine cover and make a visual inspection around the engine, including the electrical system and your fuel tank, and note anything that looks incorrect or out of place, such as disconnected hoses, wires and poor electrical connections. Take a close look at the condition of the air filter to see if it is clogged or dirty. Also check your fuel supply hoses to make sure that none are crimped or kinked.

Your boat’s environment, the water, is a natural source of rust and corrosion for wires, connectors, and anything made of metal, so check those items carefully. Make notes about anything you see that could be an issue.


Misfires and rough idling are difficult problems to isolate, since they can be caused by many system malfunctions. Ignition systems, including spark plugs, plug wires, ignition coils, and ignition timing are major players, as are lean air/fuel mixtures and mechanical issues within your engine. In older or well-used engines, worn piston rings, worn or broken valves, and worn cylinder walls, as well as worn cam lobes may contribute to the problem.

Another issue that is especially difficult to track down and identify are vacuum leaks in the intake system that can cause misfiring. 

What to Do

Misfiring may be a result of a combination of factors making a quick and easy diagnosis extremely difficult. Your best bet to identify and correct the problem of misfiring may be to schedule an appointment with your favorite marine engine technician who has the tools and the necessary experience to do the job.

Your notes about your engine analysis will come in handy in accurately explaining the problem to your mechanic, giving him/her a head start on the diagnosis and getting you back on the water at full speed as soon as possible.

Spark plugs may plan an important part in your engine diagnosis. By removing the spark plugs, a trained mechanic can get good clues as to the cause by ‘reading’ the firing ends of the plugs. The color, wetness, and physical condition of the plug tips, will give your mechanic hints about the cause—whether it’s electrically related, fuel/air mixture related and which cylinder is misfiring.  

If the resolution of your misfire requires new parts, such as spark plugs and/or an air filter, insist on brand name parts from manufacturers who have a long history and have established a reputation for brand quality, such as Champion spark plugs and filters. You should also make sure that the parts you buy are genuine, not low-budget knock-offs. 


Once the misfire or rough idling has been alleviated with quality parts, you have no worries except what to do with all your time out on the water—not a bad problem to have.

Learn more about marine maintenance products, find your boat part, or find where to buy your boat part today.

The content contained in this article is for entertainment and 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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