Bad Spark Plug Symptoms and How to Correct Them

car bad spark plug

If your car is running rough, you’re probably hoping and praying it’s something simple. Luckily, bad spark plug symptoms are easy to diagnose by yourself. If your car is acting up, it’s an excellent place to start.

You don’t need to be a mechanic to know when your car needs a tune-up. If you own a car, you should learn how to do the maintenance yourself.

By maintaining your car, you could save a pretty penny and keep your vehicle running like a champ. What’s more, you will have an easier time trading or selling a car with a good maintenance history.

As important as all this is, it’s easy to put it off. We all have busy lives and a lot of other things to remember. Changing out oil, filters, and spark plugs probably won’t be high on your list of things to remember.

That’s why we experience bad spark plug symptoms. With regular maintenance, we would be able to avoid them entirely, but many of us don’t bother.

Fortunately, a lot of cars have reminders to let you know when it’s time to change your oil. They don’t have the same kind alert for your spark plugs and wires. That’s part of the reason most people end up with bad spark plug symptoms.

They don’t usually think about changing them until their car is sputtering for help.

According to my backyard mechanics, when your car has the jitters, it’s time to check the basics. That includes fuel, spark, and air.

If you keep on top of these little issues, you will know when more significant problems arise. You can finally avoid bad spark plug symptoms and change those suckers out yourself.

What Are Spark Plugs and How Do They Work?

Motor engine block

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Spark plugs have been around since the 1800s, although there is some debate about who officially invented them. Either way, they became an essential part of most gasoline engines and are used to this day.

A spark plug is an electrical device that sits in the head of some internal combustion engines. It has two jobs: to create a spark for combustion and to pull unwanted heat away from the combustion chamber.

The combustion process

The spark plug sits at the top of the cylinder head. Depending on your engine, you will usually have between four and eight cylinders.

The piston will travel down the cylinder and draw in a mixture of fuel and air. Then the piston will move back up, compressing the mixture. Once the piston goes up as far as it can, the spark plug will produce the spark that ignites the fuel-air mixture.

That will cause a mini-explosion in the combustion chamber that will force the piston back down — in turn, creating power for the engine.

The piston will continue on its path up and down. It will clear out the exhaust fumes, draw in more fuel, and spark in time to repeat the process.

Bad spark plug symptoms are impossible to ignore for long because they affect the combustion process. Without a spark, there’s no combustion, and without combustion, there is no power. At least, not if you have an engine that requires spark plugs to operate.

Heat exchange

Spark plugs can come in hot or cold variations. Hot plugs are more common in your everyday vehicle. Cold plugs are better for high-horsepower and high compression engines.

The difference between hot and cold plugs depends on the insulation. Every spark plug has insulation to help transfer heat away from the engine and into the cooling system.

Hot plugs have more insulation that enables them to burn away carbon build-up. In turn, that helps them last longer.

Cold plugs have less insulation and pull more heat away from the engine. That is why they are used for high-combustion engines that can get hot very quickly.

Parts of a spark plug

Spark plugs look simple enough from the outside. However, a lot is going on in this tiny package.

The terminal is at the very top of the plug, and that is where the wire will connect the spark plug to a power source. Each plug has a copper core that is surrounded by an insulator. The copper core runs down the center of the spark plug, from the top terminal to the center electrode on the bottom.

In the center of the plug, there is a hex head and threads. They will allow you to loosen or tighten the spark plug in the cylinder gently.

On the very bottom of the plug, you will find the center electrode and the ground electrode.

The ground electrode hooks over the center electrode, and there is space between them. That is where the spark will jump from one electrode to the other and ignite the fuel-air mixture, creating combustion.

How long spark plugs will last

Many people are wondering how long you can go before you start experiencing bad spark plug symptoms. In other words, how long do they last?

It’s going to depend on the engine and the type of spark plugs. There are four spark plug materials on the market.

Copper spark plugs are the most affordable and are by far the weakest. They don’t last as long because they are relatively cool plugs. That means they don’t burn away carbon buildup as quickly, so they usually last around 20,000 miles.

Platinum and double platinum plugs are tougher, hotter, and last longer. They have an easier time burning away the carbon buildup and can last up to 100,000 miles. Nonetheless, you shouldn't usually wait that long.

Iridium is not as common, but they’re supposed to be the toughest on the market. Some particular models claim to last up to 120,000 miles, although it’s hard to say whether that’s true.

If you want to know which spark plugs you should use, you should check your owner's manual. It will also let you know how long you should go between changes.

That probably wasn’t the answer you weren’t looking for, but every car is different, and it’s hard to say. Most cars need new spark plugs every 20,000 to 60,000 miles. It’s best to refer to your owner’s manual.

Even if a pack of spark plugs claims to last for 100,000 miles, you should double-check at the recommended time. You might be surprised how worn out they are after only 50,000 miles.

Bad Spark Plug Symptoms

spark Plug on white surface

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How do you know when you’re having bad spark plug symptoms? Well, they’re usually pretty hard to ignore.

The first thing you will notice is the rough idle. In other words, the RPMs may jump around while it is idling, and the engine will vibrate unevenly.

Next, your engine could start misfiring. A misfire happens when the plug fails to ignite the fuel-air mixture properly. That will cause your engine to skip a beat here and there and can also affect acceleration.

When you have a misfire, the vehicle may be hard to start, and, in time, the engine won’t turn over at all.

If you ignore bad spark plug symptoms, you may cause stress on the starter and the battery. Eventually, it will stop running altogether, and you will have to do more work to find all the issues. That will lead you to spend more money replacing parts that would have lasted longer with proper maintenance.

Keep in mind that one faulty spark plug can cause engine failure even if it is the only one corroded or with an incorrect gap.

The most notable of the bad spark plug symptoms is the increased fuel consumption. When your engine isn’t running correctly, your fuel economy will take a hit. Bad spark plug symptoms aren't the only thing that will waste your precious gas, but as you'll see, it's one of the worst.

Under-inflated tires

When your tires are low, it can hurt your fuel economy on an average of 0.6 percent and up to 3 percent. That can add up to about 0.2 percent for every 1 PSI lost.

Using the wrong oil

Motor oil is meant to reduce the friction in your engine. Using the wrong oil can bring your fuel economy down. That can result in a drop of up to 12 percent.

Bad air filters or sensors

When you have clogged air filters, your car will not be getting perfect air and fuel mixture. Also, when sensors are bad, they will not be able to determine the ideal ratio, to begin with. These issues could result in a drop in fuel economy of up to 20 percent.

Bad spark plugs

Spark plugs are in charge of igniting the air and fuel mixture in the engine. When they aren't in good condition, it can cause misfires and uneven idling. That can result in a drop in fuel economy of up to a whopping 30 percent!


To keep your engine running correctly, you need to replace the filters, plugs, and oil. By keeping up with all the regular maintenance, you will preserve your fuel economy and keep your car running reliably.

How to diagnose bad spark plug symptoms

When you’re having any of the bad spark plug symptoms we mentioned, it’s time to check out your plugs. They might be the issue, and they might not. No matter what, it’s a great place to start.

After all, spark plugs and wires are incredibly affordable in comparison to many other possible parts. Plus, you will be able to tell whether you need to replace the plugs by pulling them out individually.

We already went over the anatomy of a spark plug earlier. That information is going to come in handy when you’re trying to determine whether they need to be replaced.

First, you can look at the white part of the plug. If the ceramic casing is cracked or looks burnt, it needs to be replaced.

Next, you should look at the electrodes, which are on the bottom of the plug. They need to have the proper amount of space between them for a strong spark.

The gap is the distance the spark needs to travel between the electrodes. Improper gapping can cause the vehicle to have a rough idle or misfire, even when the plugs are new.

You can check your owner’s manual to learn which spark plugs you need and what gap they should have.

If you take the spark plugs out and the electrodes aren’t appropriately gapped, they may only need to be re-gapped. If they look corroded, bent, or worn down, it’s safe to say that they need replacing.

Other issues that can cause similar symptoms

If you think you are experiencing bad spark plug symptoms, you might be right. However, if you replace the plugs and continue to have the same issues, there are a few other possibilities.

For one thing, if you replace the plugs and re-use the wires, you may need to go ahead and replace them as well.

Once you’ve ruled out bad spark plug symptoms, you can move on to other possibilities.

If your car is stock, most issues usually turn out to be regular maintenance needs. To quote all the backyard mechanics in my life, “It’s usually fuel, spark, or air.”

I have experienced issues with all three, and I have to say that they do feel pretty similar in the beginning.

Spark plugs can cause the vehicle to have a rough idle, and so can many other things.

My automatic vehicle used to stall out when I put it in reverse or idled for too long. Luckily, my dad is the ultimate problem solver, so he knew we had to replace the brake booster.

I’ve also had a bad fuel pump and a severely clogged fuel filter on separate occasions. That can cause your car to run rough, have an uneven idle, and eventually, stop running altogether.

The moral of the story here is that bad spark plug symptoms are similar to a lot of other problems. But they aren’t the only thing that can cause issues.

Nonetheless, it is a fantastic place to start. It only takes a few minutes to check them. Then you can replace them or move on to the next possibility.

Can You Replace Your Own Spark Plugs?

old spark plug

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Everyone with a car can, and should, change their own spark plugs. It’s such a simple thing to do, and you don’t need to know much about cars to accomplish it. You only need the right tools and a good tutorial.

However, changing spark plugs isn’t something you should do unless you know what you’re doing. In other words, you should learn the whole process before getting started.

I always helped my dad when I was little, so when I got my own truck, I decided to jump in and replace them myself. So I went out, got the plugs and wires, came home, took everything apart...and then I realized my mistake.

I had taken all the wires off at the same time. That meant I had to figure out how to put them back on correctly. Otherwise, it was never going to run again.

My now-husband stood out in the rain with me for hours looking up diagrams on our phones. It was such a mess.

So yes, you can absolutely change your own spark plugs. But make sure you remember all the steps before you dive in.

Otherwise, you’re going to cause an unnecessarily large headache. Plus, a project that should take half an hour could end up taking all day. Although it is undoubtedly a lesson you won't ever forget.

How to Replace Your Spark Plugs

We are going to walk you through the process of replacing your spark plugs. Please remember that every vehicle is different. Therefore, you need to do your research and find out specific details for your car.

That includes where the spark plugs are located, what type you need, and what gap to use.

Your car or truck may have a different layout than the one in the video tutorial we will link below. Our goal is to give show you the process and give you tips and steps to follow.

Step 1: Picking the right spark plugs

Step 2: Gathering your tools

Step 3: Prepping your spark plugs

Step 4: Replacing your plugs

Step 5: Check to make sure everything runs smooth

Say Goodbye to Bad Spark Plug Symptoms Once and For All

metal spark plug

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Changing spark plugs is something that you can do, no matter how much (or little) experience you have with cars. My dad was teaching me how to do it when I was little. Although I definitely should have brushed up with a tutorial before I did it myself.

Either way, you can use these tips and tricks and learn from my mistakes.

Our goal is to give you more information so you can be a confident car owner. A considerable part of that is knowing how to maintain your vehicles properly.

Don’t forget; this is one thing that you can cross off your maintenance list. You will most likely have to do it again in about 20,000 to 60,000 miles. It all depends on your vehicle.

Just remember to follow the recommendations in your owner’s manual. Don’t wait for it to become a problem before you swap out your plugs, change your oil, or get new filters.

Those recommendations are there to help you keep your vehicle in tip-top shape. Changing your spark plugs will create less wear on the battery and starter. Just like changing your fuel filter will save your fuel pump.

If you’re familiar with bad spark plug symptoms, we would love to hear about your experiences in the comment section below!

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