What Are The Symptoms of A Bad Spark Plug On Outboard?

Spark plugs ignite your outboard engine. The air and fuel mixture inside the combustion chamber gets ignited by a spark plug. They produce a small spark to make your engine start and the outboard move.

A bad spark plug can cause a halt of the outboard.

In this article, the symptoms of a bad outboard spark plug will be discussed in detail. Read all the way to the end to keep your vehicle safe from disasters!

What Sign You Should Look For To Detect A Bad Spark Plug On Outboard?

Different people will tell you different symptoms they suffered. But there is one distinctive symptom of a bad outboard spark plug.

The most obvious sign of a bad spark plug is a hard-to-start engine and sputtering sounds. That is, jerking of the outboard.

It may be mistaken for a low battery if your fuel is up! But some simple investigations can lead to a bad spark plug.

What Are The Symptoms Of A Bad Spark Plug?

Spark plugs are made of a metallic conductor that is insulated. The top end of the plug connects to the spark plug wires. From here, signals are sent along with the conducting material of the wires.

As the plugs are designed to carry electrical responses, a failure will cause an insulating effect. Everything will muffle down, one example being the acceleration.

Some symptoms of a bad spark plug are written below:

1. The Engine Doesn’t Start

To start the engine, you need the spark of a spark plug. Literally! A small jolt of electric spark is what you need to cause combustion in the engine.

The production of this spark is very calculated. The ignition coil at a fixed time sends the signal to create an ignition spark. The spark plugs carry this signal in the form of electricity.

So, when the spark plugs are fried, you face a hard start. Meaning the engine fails to jump to a start. The engine has to stress more as the spark plugs no longer support them.

2. Engine Misfires

The engine misfires because it’s not firing correctly. Bad spark plug causes incomplete burning of fuel. This is obvious as enough combustion does not occur.

Because of that, spluttering or stumbling sounds occur. Sensor malfunction occurs in this case, especially in modern cars.

3. Engine Suffering from a Rough Idle

The engine will jitter during idle. You can tell by the vibrations you will feel when running at idle.

Sometimes you may not get vibrations, but the engine will be loud.

It happens as faulty spark plugs. Engine misfires due to faulty spark plugs produce unburnt fuel. This fuel confuses the O2 sensors.

Due to this confusion, fuel escapes from the cylinder causing a lean mixture.

In this situation, a rough idle causes spike in RPMs. The higher the revs go, the higher is the pitch of the sound. And vice-versa.

The jumping is actually the compensation for the problem.

4. High Fuel Consumption

You may notice that more fuel is being taken per mileage. As the engine needs to compensate for lack of combustion, it takes more fuel.

Moreover, the gas consumption increases if the spark electrodes are too close or too far apart.

5. Low Acceleration

When spark plugs go bad, the accelerator loses its sensitivity. The ignition system is the main reason behind any acceleration issue.

In modern cars, impulses fail to be carried on time due to faulty spark plugs. As a result, the vehicle loses acceleration. It fails to accelerate on time as well.

6. Fuel Smell from Exhaust

The fuel leans out in the combustion chamber due to bad spark plugs. The unburnt fuel seeps through the exhaust. This is why a strong smell of the fuel may be found from here.

7. ‘Check Engine’ Sign is On

Your engine can be maintained recently, but the ‘Check Engine’ sign can be on.  This is a warning sign that other things are wrong with your engine.

8. Carbon Build-up of the Wires

Bad spark plugs have black, chunky build-up on them. This is carbon build-up. It occurs due to unburnt fuel in the cylinder.

These form on the ends of the spark plug. However, they do not form on the gaps.

To sum it up, spark plugs can stop all electrical components when they fail internally.

How Should You React If You Detect Bad Spark Plug?

If a spark plug goes bad, you need to replace them. But you can maintain a sequence of guidelines for the good health of the engine. These also work as safety precautions.

These are:

Cleaning the Spark Plug

A buildup of dark-colored carbon is often seen on the spark plugs. This residue comes from the unburnt fuel.

As spark plugs are very delicate, only soft brushes should be used to clean them.

Below the carbon residue, a greasy or oily residue can be found as well. Warm soapy water should be used to clean this film.


Replacing the spark plugs is easy and inexpensive. Remove the spark plug in this order: remove the spark plug, then the COP.

Then unscrew the spark plug and put in the new one. The new spark plugs may need gapping before putting in. Install the new COP. Then start up the engines.

Check the vehicle in idle. Check for spluttering sounds coming when in idle. If not, you are good to go.

Is Replacing A Bad Spark Plug Expensive?

Replacing a bad spark plug is pretty inexpensive. However, the model of the vehicle may be a factor in that.

Another contributing factor is the number of cylinders. The number of cylinders is proportional to the number of spark plugs required. A dual spark plug system should also be taken into account.

Each spark plug should cost something below or exactly $10 only. The cost for professional service should not exceed more than $150. The minimum cost for a mechanic should be around $40.

But professional help is needed when all the spark plugs are damaged in a system. Replacement of all of them requires some intricate detailing. Such as maintaining the gap between spark electrodes.

A less complex problem can be easily dealt with at the home garage.

How Often Should You Check Or Service Your Spark Plug?

Like any other vehicle part, spark plugs don’t last forever. Usually, the lifespan for spark plugs is 30,000-80,000 miles. This is universal for most vehicles.

It is ideal to check the spark plugs for every 30,000 miles. If possible, they should be replaced then.

If you see carbon build-up, first clean it and then replace the plugs.


Bad spark plugs cause unburnt fuel, bad fuel economy, and leaning of fuel. Unburnt fuel is very bad for the environment.

An engine running on unburnt fuel can be very dangerous. So it is necessary to regularly service the spark plug after a definite interval.

The spark plugs should not be used after their shelf life. And they should be replaced once symptoms of a bad spark plug are noticed.

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