Bad Spark Plug Symptoms [Learn What They Are]

Bad spark plug with soot

Not sure if your car problems are being caused by a bad spark plug? We’ll show you how to tell if a spark plug is bad so that you can take corrective measures.

If you only own an electric car, a spark plug is the last thing you will ever have to worry about. For everyone else, when was the last time you changed your spark plugs? Do you even know what they are?

These little devices play a simple but critically important role in gas and diesel-powered vehicles, providing the spark of electricity that ignites the mixture of air and fuel in their engines to get you up and moving. Your vehicle won’t start without this spark.

Fouled or faulty spark plugs can, therefore, cause a host of problems for an engine, preventing it from running optimally, if at all. If you’re looking for signs that indicate they are the cause of your car’s woes, keep reading.

Bad Spark Plug Symptoms

Here are seven signs of bad spark plugs you should take note of. See which ones apply to you and take immediate action to address them so that they don’t compromise your vehicle more than they already are or lead to expensive repairs.

1. Engine Misfires

Many things can cause an engine to misfire (poor fuel quality, a bad ignition coil plug, etc.); however, the main culprit is usually a dirty, worn, or damaged spark plug. 

You will notice engine knocking, a loss of power and other performance issues, strange intermittent engine sounds, and carbon deposits on the spark plug. You may even experience your car shaking when accelerating.

Engine misfires can cause raw, uncombusted fuel to be sent into the exhaust, which can damage the catalytic converter and ruin your engine. Don’t take it lightly.

2. Poor Fuel Economy

A sudden increase in fuel consumption or a drop in fuel efficiency is a common symptom. That’s because faulty spark plugs make the engine work harder to combust the fuel being fed to it, resulting in wastage.

Your car’s CO2 emissions may also increase, and you may even notice black smoke coming out of the exhaust

Needless to say, deteriorated plugs are not good for the environment, and you will feel their effects on your finances every time you fuel up at a gas station.

3. Poor Acceleration

If your car is not accelerating properly or as quickly as it once did, it may likely be due to a fouled or dirty spark plug.

Chances are your engine isn’t getting the energy it needs to get-up-and-go because the spark plug is no longer effective at creating the spark needed to generate that energy.

4. Engine Knock

Knocking occurs when fuel burns unevenly in the engine’s cylinders, usually due to the formation of explosive pockets caused by the failure of the spark plug to combust some of the air/fuel mixture.

It’s a serious problem that, if not addressed, can cause significant damage to the piston head and compression rings, cylinder head and valves, and even the car’s lubrication system.

5. Hard Starts

A hard start refers to when a car takes longer than normal to start. While the problem is commonly caused by a dead battery or an empty fuel tank, faulty spark plugs are sometimes the culprit.

Basically, if your car’s plugs can’t produce a sufficient spark to start the engine’s combustion process, you will experience hard starts.

6. Rough Engine Idling

If your spark plugs provide a weak or intermittent electric spark, they will cause the engine to idle roughly. You will hear a jittery sound, and the entire car might even vibrate. 

7. Engine Backfire

An engine backfire (afterburn) occurs when the combustion process takes place outside of the engine, particularly in the exhaust system. Often caused by a fouled spark plug, it’s a sign that too much fuel is going into cylinders.

When To Replace Your Spark Plugs

You should replace your spark plugs once you determine they are faulty and no longer suitable for operation. Otherwise, they are very durable devices that can last years before needing to be replaced.

Related: Learn The Causes Of Metal Shavings In Oil

Granted you should replace the ones in your car as often as specified by your car’s manufacturer, it is generally recommended that spark plugs be changed every 30,000 to 90,000 miles (48,280-144,841 km). 

Copper spark plugs are the least durable type of spark plug and fall on the lower end of that range, while platinum and iridium plugs have a notably longer operational life.

Replacing Your Spark Plugs

Left alone, faulty spark plugs can inconvenience you and potentially damage your engine. Fortunately, unless you have some kind of exotic supercar that requires highly specialized plugs and professional installation, having them replaced is fairly inexpensive.

The cost of replacing your spark plugs will depend on the make of your vehicle, the quality of the spark plug used, and whether or not you’re replacing them yourself. 

A single spark plug typically costs anywhere from $2 to 20, but some vehicles require more plugs than others. 

Generally speaking, the more cylinders your engine has, the more you need. Vehicles with dual ignition systems also require more plugs than those with regular ignition setups.

Of course, if you decide to have a mechanic or technician handle the replacement, you’ll have to pay not only for the spark plugs but also for labor, which can cost upwards of $150.

Now, should you do it yourself or take it to a pro? The answer will once again depend on the type of engine you have. 

With some engines, the spark plug can’t be replaced without first removing portions of the intake manifold — a task that’s often best left to a professional. If your engine doesn’t require this kind of manipulation, you can probably handle the job yourself.

What Spark Plugs Do I Need?

It’s often best to replace your spark plugs with the ones specified by your car’s manufacturer. If you can’t find an exact model, go with a similar one that’s high-quality and compatible with your vehicle.

There are several different types of spark plugs. While some are certainly more durable and deliver a better spark than others, there really isn’t a bad type. 

Copper sparks plugs are the oldest variation and have the shortest operational life, but they work fine in the vehicles they were designed for and can last several years if properly maintained. 

If you’re looking for a quality set of copper spark plugs, consider the four-pack NGK 4929 Standard Spark Plug. It is well-designed, durable for a copper plug, very affordable, and works on just about any vehicle.

Platinum plugs are the next level up. Constructed from platinum, a harder and sturdier metal than copper, they last longer, retain their edge better, and tend to run hotter, which helps prevent fouling and the buildup of deposits.

Double platinum spark plugs are even more durable and efficient than regular platinum plugs. The NGK 7090 G-Power Platinum Spark Plugs are a good option for vehicles that use single platinum plugs, while the Denso 4504 Platinum TT Spark Plug is one of the best double platinum spark plugs on the market.

Iridium spark plugs are a very new innovation. Harder and slightly more efficient than platinum plugs, they have a reputation for continuing to fire under extreme conditions.

If your vehicle manufacturer recommends iridium spark plugs, loosen up your purse strings and pay the price premium associated with them. The NGK 6619 Iridium Spark Plugs and Bosch 9607 Double Iridium Spark Plug are popular picks.

We have a detailed guide that reviews 10 of the best spark plugs currently on the market. Read it!

Maintaining Your Spark Plugs

Modern spark plugs are very durable and have a long operational life, but they still need to be maintained to ensure your car always has the spark it needs to startup and run. Here are some tips for keeping them in tip-top shape.

  • Always inspect your spark plugs as part of your routine vehicle maintenance regimen, checking for cleanliness and damage.
  • When replacing your spark plugs, clean the old plugs and their surrounding area before removing them. Dirt, debris, or other contaminants on them can enter the engine’s cylinder bore and undermine your engine’s performance.
  • Make sure to check your new spark plugs for damage before installing them. They should be free of dirt, water, and other substances, and their electrodes should be straight and properly aligned with each other.
  • Apply a dielectric coat to the plug’s porcelain insulator and secondary wires to prevent corrosion.
  • Coating the spark plug thread with an anti-seize lubricant will make the plug easier to remove down the line. Any lubricant that gets on the electrodes should be wiped off with a clean rag and a carburetor cleaner.
  • Only remove or install the spark plugs by hand using the appropriate wrench and socket tools. Air tools can damage them.

Spark Plug FAQs

Looking for more information about spark plugs? These questions and answers should provide you with additional useful information.

Why Do Spark Plugs Fail?

Spark plugs can fail for a number of reasons, including chemical contamination, incorrect heat ranges, improper gapping, excess carbon deposits, and of course, lifespan expiry. 

However, excessive or improper torque during installation is the number one cause of spark plug failure, accounting for approximately 90 percent of all claims.

How Long Does It Take To Change Spark Plugs?

Changing spark plugs can take anywhere from an hour to five hours depending on the number of cylinders the engine has and your skill level. For most engines, the replacement is a fairly simple and straightforward process that only requires a bit of elbow grease.

How Much Does It Cost To Change Spark Plugs?

The typical spark plug costs between $2 and $20, and you likely won’t have to spend more than that if you perform the replacement yourself.

With professional installations, you can expect to pay anywhere from $40 to $150 for labor, on top of the cost of the plugs.

Can I Throw Away Old Spark Plugs?

Getting rid of old or faulty spark plugs is a lot easier than, say, disposing of old gas. You can throw them into the garbage without serious health repercussions to people, animals, or the environment or have them recycled.

Auto scrap is valuable, so saving up your used spark plugs and having them scrapped instead of thrown out can get you some money. 

Final Thoughts

It’s the spark plugs that provide the electric spark your engine needs to combust and run. Without them, your car won’t start, plain and simple.

Like any other car part, these devices age and break down over time and can be very problematic when they do. The most common signs of bad spark plugs are a drop in acceleration and fuel economy, engine knocking and misfires, hard starts, and rough idling.

Being aware of these symptoms and taking immediate action to address them will save you a lot of headache and money. That last thing you want is for a faulty spark plug to cause irreparable damage to your engine.

If you found this article about bad spark plug symptoms helpful, you might also be interested in knowing why your turn signals only work sometimes, how to fix a sagging headliner, and how much the average car weighs.