6 Symptoms of Bad Spark Plugs – & Replacement Cost

A spark plug is an essential part of a gasoline engine where they play a vital role in the combustion process, responsible for igniting the fuel and air mixture that powers the engine. But over time, spark plugs can become fouled or damaged, leading to several problems you’ll want to address as soon as possible to avoid further damage to any other parts of your vehicle.

In this article, we’ll discuss some of the most common symptoms of bad spark plugs and also how to tell if spark plugs need to be replaced to keep your car running smoothly.

Let’s get started.

6 Symptoms of Bad Spark Plugs

Changing the spark plugs should be a part of your car’s routine maintenance. But if they become damaged or don’t get replaced, you may notice some of these bad spark plug symptoms listed below.

But keep in mind that it can be other issues causing the same symptoms. So with that said, make sure to always determine the cause before replacing any parts.

1. Decreased Fuel Efficiency

Been making more trips to the gas station lately? Did you know that bad spark plugs can reduce fuel efficiency by as much as 30%? So if your car is suddenly guzzling gas more than usual, chances are dirty or fouled spark plugs are to be blamed.

If troubleshooting the cause is nothing you want to do, your mechanic will be able to tell if the spark plugs need to be replaced. And once your vehicle has new spark plugs, you’ll notice a difference in fuel efficiency and how much better the car feels while driving.

2. Engine Misfires

If a spark plug is fouled or damaged, it may not be able to provide the spark that is necessary to ignite the fuel in your engine. When this happens, your engine will misfire.

Misfires are often the culprit of many of the listed symptoms in this article. Not only that, but they can also increase emissions and damage your catalytic converter. In severe cases, it can even cause engine damage.

A few signs of misfiring that can’t go unnoticed is sputtering sound from the car engine, violent shaking or jumping, or sudden drops in engine power even when holding your foot steady on the gas pedal.

You’ll also often find the check engine light illuminating your dashboard as the car misfires. A flashing or blinking check engine light indicates that the more you drive, the more damage you possibly cause to your engine. And if the check engine light is flashing, you should immediately stop.

3. Rough Idle

If the engine sounds rough and uneven instead of smooth and steady when idling, it could be a sign that the spark plugs are worn or contaminated. When that happens, they can’t create the right amount of sparks, leading to a rough idle.

A rough idling engine can also feel like your car is shaking slightly, and you may notice the RPM fluctuating up and down on your tachometer (rpm gauge), and it may even feel like the car is about to die.

The most important thing is that the idle speed is steady. The average idle speed of most of today’s cars is around 700 to 800 RPM. But anywhere between 600 and 1000 RPM should be seen as normal.

4. Trouble Starting

If you’re having trouble starting your car, it could be due to many different issues. But if you are experiencing longer cranking times, especially on cold mornings, you’ll want to take a look at the spark plugs. Over time, worn spark plugs and carbon buildup from unburnt gases and oil will eventually make it harder to cold-starting your car.

If you need to crank the engine for too long without starting, you’ll eventually have a drained battery. And you’ll also cause more load and wear on other parts of your car.

5. Acceleration Problems

Acceleration problems go hand in hand with engine misfires. But you might not notice the misfires as much until you put more load on the car, such as when accelerating or going uphill.

When you press the gas pedal, you tell your car to provide more fuel to the combustion chambers, but with bad spark plugs not properly igniting all this fuel, the irregular ignition will cause your car to hesitate.

6. Knocking Sounds From Engine

While this symptom isn’t as commonly known and doesn’t show as clearly as some of the others above, you can sometimes hear your engine making a distinct knocking sound because of bad spark plugs, especially while accelerating.

This happens due to the spark plugs not igniting the fuel properly, which causes uneven burning of fuel and small “explosions” in the combustion process.

But no matter the cause of these sounds, if you hear knocking from the engine, it’s always something you immediately want to address. If left unchecked, it can quickly cause serious damage to vital parts of your engine.

The good news is, if the spark plugs are at fault for the knocking sounds, it’s a simple fix. However, if the knocking sounds are caused by something else, you may end up with expensive repairs.

How to tell If Your Spark Plugs Need to Be Replaced

If you’ve experienced any of these symptoms above, the easiest way to tell if your spark plugs need to be replaced is to do a visual inspection and compare the old spark plug to a new one. But to do this, you’ll need to know how to remove them.

Look at your spark plugs and look for any excessive wear to the tip of the plug; if you see any wear, it’s time to replace them. Also, make sure to look for any fuel or oil contamination.

If there is any contamination, it’s not only an indication that you need to replace the spark plugs but also that something is causing the contamination.

Oil and fuel contamination on spark plugs are both symptoms of failing components inside the engine itself and should be addressed as soon as possible to avoid further damage to the engine.

If you’re unsure of what you’re looking for, it’s always a good idea to consult a professional mechanic who can take a closer look and give you an expert opinion.

How Often Should You Replace Your Spark Plugs? 

The frequency with which you need to replace your spark plugs depends on a few factors, including the engine type in your car, the conditions in which you typically drive, and the brand of spark plugs.

The recommended spark plug replacement intervals usually range from 30,000 miles to 100,000 miles, depending on the car manufacturer and the spark plug metal type and design. However, they can become damaged or fouled earlier than expected for several reasons, meaning you may have to replace your spark plugs more often.

Make sure to consult your owner’s manual or a mechanic before deciding on which brand of a spark plug to buy. Upgrading is OK, but you should NEVER downgrade the spark plug from the car manufacturer’s requirements.

Here are a few things that can cause a spark plug to fail earlier than expected:

  • Oil leaks into the combustion chamber
  • Overheating
  • Carbon buildup
  • Improper spark gap

Cost to Replace Your Spark Plugs

The cost to replace your spark plugs will vary depending on the type of car, metal type, and brand of plugs. Generally, a spark plug replacement at a mechanic will cost between $110 and $200.

The good news is that around half of this comes down to labor costs. The spark plug itself is an inexpensive part that usually costs between $5 and $20 each, depending on the type. There is one plug to each cylinder in your engine, and most cars have four to eight cylinders.

On most cars, it’s not a very particularly difficult or labored procedure to replace the spark plugs, so you can expect a mechanic to charge for one or two hours of labor. So doing the work yourself is one way to keep down the spark plug replacement cost.

How to Replace Your Spark Plugs

Replacing spark plugs is a relatively simple process on most vehicles that can be done at home with just a few basic tools.

However, on some vehicle engines, you may find it a bit more challenging due to accessibility. It can also require removing the upper plenum/intake manifold to replace the spark plugs. In this case, you’ll need a new plenum gasket.

So if you’re unsure whether or not you can do it yourself, always consult a mechanic.

Here’s a great step-by-step guide on How to Change Spark Plugs by AutoZone.

Final Thoughts

Now that you know the typical bad spark plug symptoms, you should have the necessary information to tell if your spark plugs need to be replaced. It’s often an inexpensive job, and neglecting to replace bad spark plugs could lead to bigger problems down the road and may even damage your engine beyond repair.


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Rickard Cefalk

Rickard is the owner of Caraspect.com and a dedicated and avid do-it-yourselfer who has always enjoyed working on his own vehicles since childhood. He now devotes his time to sharing his expert knowledge of car maintenance and other car-related information through his website.

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