4 Common Symptoms of Bad Spark Plugs on Your Outboard

Like every day, you take your boat out on the water and start cruising. Moments later, the outboard engine starts malfunctioning. You make your way back from the water and proceed to check the engine to determine what could have gone wrong with it. Is it the spark plugs?

If yes, how can you understand if the spark plug is the culprit? What are the visible symptoms of bad spark plugs outboard? The engine tends to hesitate or cut out when the spark plug is bad. Besides, you may experience loss of power and acceleration. 

However, even these symptoms might not be enough to detect a bad spark plug. And unless your suspicion turns into confirmation, a solution is unimaginable. Don’t worry we have more detailed info for you below.

What Symptoms Do Bad Spark Plugs Show on the Outboard?

When your outboard engine’s spark plug is bad or fouled, most of the effects are expressed through the engine. 

That means you must be more careful about your engine’s behavior rather than visible symptoms on the spark plug. 

Nevertheless, we’ll also talk about visible signs on the spark plug. Now let’s get into the main discussion. 

Symptom 1: Your Engine Sputters When Trying to Start

Spark plugs play an essential role in the operation of a boat engine, which creates the spark that ignites and ensures proper combustion of the fuel-air mixture to power the engine. 

When the spark plug in your outboard engine is damaged, fouled, or too old, it can’t provide proper combustion of the air-fuel mixture. 

Consequently, the engine won’t get enough power to start, which is what we call sputtering. 

Even though some other causes are responsible for this situation, it’s not a bad idea to suspect the spark plug first. 

Symptom 2: Engine Sound Becomes Harsh

Whether your outboard engine is in idle condition or you’re cruising on the water, bad spark plugs will exhibit something unusual that warns you. 

What is this? Yes, harsh sounds from the engine that you might not encounter before. 

If you have faulty spark plugs, your engine will make a rough sound or noise that resembles knocking, pinging or rattling while the engine is running. 

In some cases, you may feel your boat is vibrating unusually, which is the extreme level of the symptom. 

Symptom 3: Engine Performance Drops Drastically

Assuming that you have somehow managed to start your outboard engine even if it’s sputtering. 

However, you won’t get the optimum performance if the spark plug is damaged or fouled. The performance drop would be so prominent that you can instantly feel it, especially under heavy loads. 

Monitor the performance of your engine in rough water or accelerating, and you may also experience surge or hesitating in the engine. 

In some cases, the engine may shake or stutter abnormally, depending on the severity of the damage and varying speeds. 

Symptom 4: Visible Sign: Deposits on Spark Plug or It’s Burned

Sometimes, deposits arise on the spark plug’s electrode tip from overexposure to extreme heat. 

The excess heat melts and merges substances and creates an oblong, glazed, or shiny appearance on the spark plug. 

In general, the deposits on your spark plug due to overheating may look uneven and, most importantly, blistered in dark color. 

So, if you notice the tip of the electrode is discolored or blistered, it signifies the spark plug is overheated or burned out. 

3 Possible Causes of a Spark Plugs Failure on an Outboard Engine?

You might be acquainted with the symptoms of a bad or fouled spark plug in your outboard engine. 

But what causes spark plug failure? Do you have any ideas? Maybe not. Let’s see some of the common culprits behind the issue.  

1. Carbon Build-Up

Carbon buildup is one of the most significant causes of failing spark plugs unexpectedly. However, carbon build-up is often triggered by various factors. 

For instance, a defective heat range plug, equipment overheating, or continuous use for excessive hours. 

Sometimes, an overdone carburetor setting can also result in carbon deposition. 

In a dual-cylinder outboard, the difference between unmeasured amounts of fuel is added to the combustion chamber. A weak spark may be the reason for the phenomenon as well. 

Greasy deposits or dark, oily liquid on the tip of a plug’s conductor and insulator represent the possibility that oil has leaked out of a cylinder. 

You must find the leak to determine where its source is. Give it your mechanic’s opinion so that the issues are repaired.

2. Choosing an Incorrect Spark Plug

Most often, a fouled spark plug may appear even after the engine is started properly and lets you cruise for a while. 

However, after a certain period, it may start losing speed and RPM and even turn off at the end. 

If you have recently changed your outboard engine’s spark plug and are experiencing such incidents, check whether an excessive amount of gasoline is flooding the engine. 

In such instances, choosing an incorrect spark plug is mostly responsible for the issue. So, consult your mechanic before replacing your spark plugs. 

3. Failed Cold Start Enrichment Circuit

A failure in the choke or cold start enrichment circuit can also be responsible for getting the spark plug fouled. Before the breakout of serious repairs, other parts may fail, costing a lot of money.  

If you plan to purchase a two-stroke outboard in the vicinity, you should look at client testimonials and speak with others who know of the engine.

Replacement and Repair Costs of Outboard Bad Spark Plugs

Outboard engines can be either two-cylinder or 4-cylinder. At the same time, you’ll also find different types of spark plugs for outboard engines. 

As a whole, the cost can vary greatly depending on your engine and the type of spark plug. 

However, for a single spark plug replacement, it may cost you approximately $60-$120 on average. Check the table below for detailed info:

Spark Plug TypeSpark Plug Cost (one)Total Replacement Cost (with Labor Cost)
Copper Core Spark Plugs$13-$15$60-$100
Platinum Spark Plugs$18-$25$70-$120
Iridium Spark Plugs$13-$18$60-$100

Tips for Maintaining the Outboard Spark Plugs to Prevent Future Failures

As you can see, different factors can cause spark plug failure in your outboard engine. 

No matter how reasonable the replacement cost is, you might not want your engine to be stopped in the middle of the water. 

To avoid any uncertain situation and maintain your spark plugs, follow these tips:

  • Check the spark plug before you head to cruise. 
  • Make sure the gasoline mixture isn’t too rich or too lean. 
  • Always choose the proper spark plug specified for your outboard engine. 
  • Prevent engine oil leaking. If there’s any leakage, repair it as soon as possible. 
  • Make sure the insulator tip of the spark plug is in fresh condition (check the signs above to identify bad spark plugs)
  • Make sure the spark plug isn’t worn out.


We have tried our best to provide all the necessary information about bad outboard spark plug symptoms. However, if you have some questions still in your mind, check the section below.

How often do you need to change the spark plugs in an outboard motor?

Boaters have a heated argument about this topic. Some boat owners insist on replacing them usually once a year or every 100 hours. We recommend regularly inspecting the spark plug and replacing it if necessary. A rule of thumb is to change them out every 200 hours regardless of their condition.

What happens if you go too long without changing spark plugs?

If you neglect to change your spark plugs frequently, you can face serious damage to your car engine. Deposits that accumulate on the old plugs will eventually lead them to fail. Moreover, the unburned fuel in the cylinders can eventually damage the internal walls of the cylinders.  

How do you know if your outboard is misfiring?

If the outboard engine misfires, you might have issues accelerating your boat. For the most part, poor acceleration occurs when the vehicle is under load. An intermittent or jerking motion may occur as you try to speed up. 

Which Spark Plugs Do You Need in your outboard engine?

Ideally, you should purchase marine-grade spark plugs rather than auto-grade spark plugs for your outboard engine. Your specific spark plugs will normally be mentioned in the owner’s manual. Besides, some engines come with a label you can reference for what plugs are correct for your particular engine.

Final Words

Spark plugs derive an ignition spark, which ignites the air-fuel mixture in the engine cylinder, allowing the engine to start. As soon as you notice the above-mentioned symptoms of bad spark plugs outboard, don’t forget to replace the spark plugs. 

While this type of component is unpredictable, your conscious mind can expand the lifespan of the spark plugs. So, before you notice any damaging signs, try to maintain the spark plugs properly following our tips.

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