6 Symptoms of Bad Spark Plugs

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Are you concerned you might have bad spark plugs?

I understand your concern; a bad spark plug can lead to a plethora of problems. Not only can it lead to worse mechanical problems, but if left unchecked can be an expensive repair.

Don’t stress any longer. In this post, I will review the seven main symptoms of bad spark plugs, so you know exactly what to look for.

After reading this post, you will know everything you need to know about bad spark plugs.

Let’s get started.

What is a Spark Plug?

Before we get started, let’s cover the basics.

A spark plug delivers the spark or power that ignites the mixture of fuel and air in the fuel chamber. 

This ignition is what gives the engine power and allows it to run.

Spark plugs are usually insulated and screwed into internal cylinders inside the engine head. 

They produce the required electricity by creating an arc of electricity between two leads that are not touching.

These leads are not touching but close enough for the electricity to create a lead.

This leads is what will eventually ignite and give the engine power.

Symptoms of Bad Spark Plugs

if you do have a bad spark plug there are some key symptoms that you will experience.  It’s important to note that a bad spark plug doesn’t always cause these symptoms. 

The symptoms you experience will depend on the type of car and how damaged are your spark plugs. 

Below is a list of the most common symptoms of a bad spark plug. 

1. Lack of Acceleration

A bad spark plug can cause a lack of accelerations.

When spark plugs are faulty and do not produce enough electrical current, this can lead to poor combustion. This lack of combustion doesn’t give your engine enough power and causes a lack of acceleration.

A lack of acceleration is noticeable when you press your foot on the gas. If you are experiencing a lack of acceleration when pressing your foot on the gas pedal, there will be a lack of power or delayed acceleration.

There aren’t many accurate ways you can measure your acceleration. Most people will recognize reduced acceleration due to familiarity with their car.

There is one tool you can use to measure the acceleration of your car. Dragon’sDragon’s are devices often used by car enthusiasts to measure the performance of your car.

You can also use this device as a casual driver to measure your car’s performance.

This device is most effective if you have measured your car’s performance before you have a bad spark plug. But it can still be effective, especially since, over time, it’s likely your spark plug’s performance will continue to degrade and cause more acceleration issues.

This GPS based performance can track your acceleration from 0-60mph, 60-130mph times, 0-130mph times, 1/4 mile times, and 1/2 mile performance times.

You can track your car’s progress over several weeks on your commute to work. Use a single stretch and determine if the acceleration is getting worse.

2. Reduced Gas Mileage

A bad spark plug can decrease your fuel economy by up to 30% and can cost you about 94 cents per gallon. 

If you experience a sudden drop in gas mileage, you may have a bad spark plug. 

An easy way to determine if you are experiencing reduced gas mileage is if you need to refill your tank more frequently. 

This is especially easy to measure if you follow the same commute every day. For example, during the week, I typically only drive to and from work. When I follow this routine, a full tank will last me until Thursday. 

If I keep to my routine and need to fill up gas on Tuesday or Wednesday, I know something affects my gas mileage. 

For a more accurate way, you can measure your gas mileage. 

In newer vehicles, the gas mileage will be displayed on your dashboard or the central display. Keep track of these over several weeks. 

You can compare your MPG to the MPG recommended for your car’s specific year and model. 

An easy way to determine your vehicle’s average gas mileage is using the U.S Department of Energy’s fuel economy website. 

If your car does not display the MPG on the dashboard, you can use an app to track your MPG. 

If your car has a lower MPG than the recommended MPG for your vehicle, I suggest you inspect your spar plugs. 

3. Engine Misfires

Another common symptom of a bad spark plug is an engine misfire. A lot of people think of a backfire when they hear the word misfire.

The truth is they are different. Backfires cause a “pop” or “thud” from the exhaust pipe. Engine misfires happen when the spark, fuel, or oxygen inside the engine are not in sync.

If you have a bad spark plug, the cause of the issue is the spark or ignition.

There is not one sign you can look for to determine if your engine is misfiring. A faulty spark plug can cause a variety of problems with your engine.

Some common issues include sputtering, thumping, or clicking noises in the engine. But, engine misfires due to a bad spark plug can also reduce engine performance.

If you feel your engine is operating outside of normal conditions, you might have a bad spark plug.

4. Engine Knock

Another common sign of a bad spark plug is engine knock. Engine knock typcially happens when the fuel inside the engine burns unevently. This causes the “knock” sounds inside the engine.

These knocks are the multiple ignition points of the fuel. When spark plugs are working properly the fuel ignites simultaneously.

Engine knock can cause serious damage inside the engine if not fixed.

After the piston head is damaged, the next to go out will be the piston compression rings.

The cylinder head may punch through if it consistently receives pressure from the combustion chamber. It could also melt the cylinder head valves.

Metal debris originating from all these knocks may spoil the lubrication system and cause the other engine parts to fail.

5. Rough Idling

Rough idling is characterized by jerking or stutters when driving or starting the vehicle. This is especially common if you are driving with a heavy load.

When you see your Check Engine Light cut on, feel a rough idle, a stalling, and a hissing sound from the engine bay – these are all symptoms of a vacuum leak.

You will feel your engine run at higher RPMs but increase, run rough, and have a difficult time maintaining stable RPMs on idle.

The RPM while idling decreases below the ideal level which results in rough idle when there is an uneven supply of fuel. You will need to restart the vehicle if the RPM falls too low because it will stall.

6. Hard Start

A hard start is when your engine takes longer to start than it should. When you have a bad spark plug, the ignition spark its producing is not enough to ignite the air-fuel combination. 

This causes the engine to sputter or hesitate before it can officially start. It some cases you may need to crank the ignition multiple times before the engine starts. 

If you car is experiencing hard starts it does not mean that you have a bad spark plug. 

There are a few other reasons that can cause hard starts. 

One of he most common reasons for a hard start is that the starter.

Below are some of the other reasons your car can be starting hard: 


  • Low Fuel Pressure

This occurs when the combustion can’t begin in the engine if the fuel injectors are not getting enough fuel. 


  • Bad Starter

The starter is a small motor that spins the pinion gear. The pinion gear engages with the engine’s flywheel and begins to spin when you turn the key in your vehicle. This is how the engine begins running. Eventually, the pinion gear’s teeth can wear down and you will have to replace or repair the starter.


  • Bad Fuel

This occurs when you accidentally contaminate your gasoline with water, too much alcohol, or diesel fuel.

What Does The spark Plug Do?

1. Ignite the fuel and air mixture

Spark plugs transmit electrical energy that ignites the fuel inside the fuel chamber.

This chamber is what allows the energy to jump the gap in the plug’s firing end if it receives a high enough voltage.


2. Seal the Combustion Chamber

The electrical power activates the gas and air mixture in the chamber. The spark plug is located at the top or on the side of the cylinder head (depending on your vehicle engine type).

When the piston travels down to the cylinder, it draws in air and fuel mixture.

The piston elevates back to the spark plug; it compresses the mixture.

While the piston is at its top dead center, the spark plug powers and ignites the mixture.

When the piston is back down, it creates power, then to clear out the exhaust, it is pushed back up again; this cycle keeps ongoing.


3. Remove Heat from Combustion Chamber

Spark plugs can remove heat but cannot create it. The plug’s firing end temperature is sensitive. It must be kept at a low temperature to prevent pre-ignition. Yet it must be at a certain temperature to avoid fouling.

Spark Plugs Conditions & Causes

Spark plugs can fail for a variety of reasons. One way you can tell cause the spark plugs to break is the condition they are in.

After you remove it from the cylinder, the condition of the spark plug can have signs of the original issues.

Below we will review some of the common conditions you may encounter for your spark plug and its causes.


Normal-reading spark plugs will work and look the same way when they were first installed in the cylinder head. The engine would be functioning accordingly.

Carbon Fouling

Carbon fouling is one of the most common reasons spark plugs fail. 

It’s important to note that carbon fouling is not caused by a spark plug. Carbon fouling damages a good spark plug causing other issues such as misfires. 

Carbon fouling causes 

One of the main causes of this issue is overly rich conditions. Overly rich condition is a sign that your engine is receiving too much fuel and not enough air. 

Overly rich fuel conditions cause carbon deposits around the spark plug. These deposits are conductive and can cause misfires. 

This can be a result of a variety of issues. 

The air cleaner is clogged or the carburetor may be the underlying problem.

There could be several probable causes: overly retarded timing, low compression, improper spark plug heat range, or a vacuum leak.


Overheating spark plug

Prolonged overheating may cause abnormal combustion. This could result in spark plug electrodes melting.

The igniter’s insulator surface turns pure white and the burning gases deposit spots when it’s overheating. 

Oil Fouling

oil fouling spark plug

The oil may be from the piston rings, valve stem seals of the Positive Crankcase Ventilation (PVC) system.

It will give the spark plug a distinct smell of the engine oil.

When the PCV valve is open, the spark plugs will quickly foul due to the excess vapors and oil droplets.

Broken Insulator

broken insultor spark plug

The temperature of the combustion gas rises sharply and the spark plug, piston, and valves will all overheat.

This causes the engine to start knocking and detonation can occur.

The aggressive nature of knocking and commotion can break the insulator, especially when the spark plug center electrode expands due to overheating.


torched seat spark plug

This happens when your spark plugs have been running too hot for a prolonged time and burned a hold through the top of the piston and melted an electrode. 

Mechanical Damage

mechanical damage spark plug

A clear indicator of the spark plug as mechanically damaged is when it appears to be beaten by its piston. This shows that it has extended too far down into the combustion chamber.

Frequently Asked Questions

How to replace the spark plugs?

Changing your spark plugs sounds a lot harder than it really is. 

Replacing the spark plugs on your car is just a little bit harder than changing a light bulb in your home. 

The main difference is that in some cars reaching the spark plug is difficult and may require special tools. 

Below are straightforward steps you can follow to replace the spark plugs on your car. 

  1. Remove the ignition cables.
  2. Disconnect the spark plug wires.
  3. Use a socket wrench to remove the spark plug.
  4. Examine the new spark plug for any damage.
  5. Add anti-seize on the threats of the spark plug before installing the new spark plug. 
  6. To help prevent misfires and make it easier to remove the boot, apply a thin coat of the dielectric grease to the porcelain and terminal of the new spark plug. 
  7. Install the new spark plug into the head.
  8. Put back the spark plug wire boot and put back all the parts that were initially disconnected.

Can you drive with a bad spark plug?

Yes, you can drive with a bad spark plug. Although it may cause a variety of problems such as low gas miles, rough idling, hard starts, and engine knock you can drive with these symptoms. 

You should not drive with a bad spark plug for an extended period. 

It’s very dangerous. This will only cause more damage to your engine the longer you put it off. Set up an appointment with your local auto repair shop immediately if you cannot replace it yourself.

What happens when your spark plugs go bad?

A faulty spark plug can affect the performance of your vehicle. This can cause failure to ignite the fuel-air mixture and cause interrupting with the engine running. It will cause insufficient combustion and impairment to the catalytic converter of your vehicle.

What causes spark plugs to go bad fast?

There are many reasons that a spark plug can go bad. Below are some of the most common reasons a spark plug breaks. 

1. Carbon Fouling

As we mentioned before, overly rich conditions are usually the cause of this. The air cleaner is clogged or the carburetor may be the underlying problem for this. Probable causes could be are overly retarded timing, low compression, improper spark plug heat range, or a vacuum leak.

2. Overheating

As previously discussed, abnormal combustion may be induced by prolonged overheating. This could result in spark plug electrodes melting. The igniter’s insulator surface turns pure white and the burning gases deposit spots when it is overheating.

3. Fuel Contamination

Occurs when you accidentally contaminate your gasoline with water, too much alcohol, or diesel fuel.

Where are my spark plugs located?

Spark plugs are located in the cylinder head above the combustion chamber.

The location of your spark plug differs depending on your vehicle type and your engine.

The cylinder head and combustion chamber are positioned differently depending on your vehicle and engine. 

Four-cylinder engines spark plugs are on the right top of the engine. On top of the valve cover, four coil packs can be found in a horizontal line. 

On opposite sides of the engine is where you will find the V8 engine spark plugs.

You will find four spark plugs on each head with two heads running.

You may also locate the spark plugs right under the valve cover next to the headers exhaust pipes. 

How many spark plugs per cylinder?

There’s is one spark plug for every cylinder the engine has. For example, a four-cylinder vehicle will have four spark plugs. A V6 will have six spark plugs while a V8 will have eight spark plugs. 

What are the four types of spark plugs?

1. Copper

Copper is the most common and most inexpensive available. The outer nickel-alloy material is not as hard so it wears down quicker with the high pressure and heat produced in the cylinder of the engine. This tends to have a shorter lifespan.

This runs cooler and produces more power performance. In turbocharged, supercharged, and higher compression ratio engines – these are installed as part of the original equipment.  


2. Iridium

Iridium has a longer lifespan (25% longer) than platinum.

It’s six times harder and eight times stronger than platinum. Iridium also has a 700-degree melting point. 

It includes fine electrodes while keeping excellent wear characteristics. These are usually pricey, this will run you from $8 to $15 each.


3. Platinum

This is similar to a copper spark plug, the difference is that the center electrode has a platinum disc that is welded to its tip.

The platinum spark plug is recommended for a vehicle with an electronic distributor ignition system. They usually cost about $16 or higher per spark plug.


4. Double Platinum

This type of spark plug is recommended for drivers with a Waste Spark System for your distributor ignition System.

The great thing about the waste spark system is that it isn’t affected by the outside weather (dampness or rain), making it more reliable.

This ignites twice, once in the compression stroke, and the second is in the exhaust stroke’s cylinder.

The price range is similar to the platinum spark plugs.

Resource: 10 Best Spark Plugs

How long should spark plugs last?

On average, Americans drive 13,500 miles per year. Spark plugs are expected to last up to 100,00 miles per year when your engine is functioning properly. 

A more realistic figure is you should replace your spark plugs every 30,000 to 70,000 miles. 

This means you should expect to change your spark plugs every two to fours years. 

If you are driving luxury or performance vehicles you may need to change your spark plugs sooner. The engines in these cars require more power and spark plugs are expended much faster. 

The type of spark plug you have will also influence how long they will last. 

Copper spark plugs can last between 10,000 to 20,000 miles. 

Platinum spark slugs can last between 60,000 miles and 100,000 miles. 

Iridium spark plugs typically last about 80,000 to 100,000 miles.

Costs of replacing your spark plugs?

The spark plug itself should run you about $16 to $100.

Copper and Iridium spark plugs typically cost less than platinum and double platinum spark plugs. 

You will need to change all your spark plugs at once so you can expect to pay anywhere from $30 to $300. 

If you have a larger engine such as a V6 or V8 you will pay more because you need more spark plugs. 

Likewise, if you have a performance or luxury vehicle you will likely need to pay more because they typically require platinum or double platinum spark plugs. 

If you decide to pay for a mechanic to install it for you, that would cost about $50 to $250 in addition to the cost of the spark plugs. 

It should only take a professional mechanic an hour or so to get the replacement done.

How do you choose the right spark plugs for your car?

Choosing the right spark plug for your car is not a hard task. 

The bare minimum you need is to maintain the manufacturer’s minimum requirement. 

If you want to upgrade then you can consider higher quality spark plugs for your car. 

Once you know the type and brand of spark finding the right spark plug is easy. Whether you are online or in-store you will need to provide the make, model, and year of your car. 

This will ensure that you are getting the right model of the spark plugs you are looking for. 

If you need some help choosing the best spark plugs for your car check out this article.

Here, I outline the top 10 spark plugs on the market regardless of the type of vehicle you drive. 


Since you’ve already made it this far, I guess it won’t hurt to ask you to get to know your spark plugs some more.

Know the symptoms of bad spark plugs so that it’s easy for you to get them replaced.

I hope you’ve learned a lot and that I have given you a better understanding of what this broad world of the spark plug is about. 

1 thought on “6 Symptoms of Bad Spark Plugs”

  1. I started my car early morning without making enough right pressure to drive I drive it & it got stuck in a snow. I tried to get out it from the snow & pressed race peddle very much or I can say 50 times. Eventually it damaged my spark plugs, my check engine light start flashing & I am facing the exact symptoms before my car spark plugs got enough damaged to get into my notice. I am looking to change it as soon as I can. Thanks for this wonderful article.


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