6 Symptoms of Bad Spark Plugs (and Replacement Cost)

Spark plugs have one very important purpose: to ignite the air/fuel mixture within the combustion chamber to start a car and keep it running. A secondary purpose is to transfer heat away from the combustion chamber to prevent pre-ignition.

Making sure the spark plugs in your car are in good condition is crucial for it to operate properly. The tiny bolt of lighting each spark plug delivers every time a piston completes a cycle (at top dead center), is necessary to ignite the mixture of compressed air and fuel.

Every cylinder of a vehicle has a spark plug so a four-cylinder car has four spark plugs, a six-cylinder has six plugs, and so on. When even one spark plug is bad or dirty, your vehicle’s performance will suffer. Fortunately, there are some symptoms you can watch for to help diagnose faulty spark plugs.

*Note that diesel engines do not use spark plugs; they use glow plugs.

Top 6 Bad Spark Plug Symptoms

As you or your mechanic are diagnosing the cause of your car trouble, there are indications your spark plugs are deteriorating to look out for. The 6 most common symptoms that your car’s spark plugs require replacement are as follows:

1) Hard to Start

Since a strong spark from the plugs is necessary to start the engine, one of the most obvious indications of faulty spark plugs is that the car struggles to start. Hard starting is often an indication your spark plugs are fouled up and need replacing.

Keep in mind that spark plugs do not usually go bad all at the same time. If your car cannot start at all, the battery may be the culprit.

2) Engine Misfires

Because a bad spark plug can cause incomplete burning of fuel in the combustion chamber, engine misfires can be the result. If you notice intermittent sputtering or stumbling sounds from the engine, it may be due to faulty spark plug or some type of sensor malfunction. In many cases, your check engine light will also turn on.

3) Bad Fuel Economy

If your fuel economy is suffering, it could be as a result of the spark plugs going bad. When spark plugs deteriorate, they lose their ability to burn fuel efficiently.

The reason for poor gas mileage is often that the gap between spark plug electrodes is either too close or too far apart when compared to factory settings. It may be possible to take out each spark plug and adjust the gap to correct specs but in most cases, new spark plugs are recommended.

4) Rough Idle

Your engine should purr like a kitten when it idles and stay at a consistent RPM. If you notice the RPM dropping or surging or the car vibrates too much, the car’s computer (ECU) may be trying to compensate for bad spark plugs. Inspect each plug and replace as necessary.

5) Poor Acceleration

If your car does not respond like it used to when you step on the gas, worn out spark plugs may be to blame. While sluggish acceleration can be due to a number of reasons, it’s always best to troubleshoot starting with the easiest things to check for, which would be inspecting spark plugs in this case.

6) Strong Fuel Smell From Exhaust

If bad spark plugs are not properly igniting the air/fuel mixture in the combustion chamber, unburned gasoline will make its way into the exhaust system. You will then notice a strong gasoline smell coming from your tailpipe.

This is in essence a condition called running rich which can be attributed to many things but again, starting diagnostics with something as simple as spark plugs is a good strategy.

Spark Plug Replacement Cost

Spark plugs themselves are one of the most inexpensive components on a car. A single spark plug will usually cost in the range of $4 to $10 depending on the type of spark plug material. But, since the amount of plugs you need is dependent on how many cylinders your vehicle has, you will need to multiple that figure by 4, 6, 8, or even 10 in some cases.

Standard spark plugs made of nickel alloy are less expensive than premium spark plugs which are made of iridium or platinum. It has been said that premium spark plugs may help fuel economy; be sure to check with your mechanic if this upgrade would be appropriate for your car.

For a typical four-cylinder car, expect to pay around $16 to $40 in parts. Six-cylinder vehicles between $24 and $60 for parts while an 8-cylinder truck or car will run you about $32 to $80 in parts.

Most mechanics will charge approximately $50 to $200 in labor to replace all the spark plugs in a vehicle. The biggest factor is the accessibility of the plugs and whether you’re going to a dealership or independent auto repair shop.

So, all together (parts and labor), expect to pay anywhere from about $70 to $300 to have your spark plugs professionally replaced. The price will largely depend on the make and model of the car, as certain cars require the best spark plugs available.

While replacing the spark plugs has the potential to be a pricey project, there is good news. Most vehicles don’t require frequent spark plug replacement and can last anywhere from 30,000 to 100,000 miles or more. Most newer vehicles have a 100,000 to 120,000 mile change interval these days.

Can You Replace Spark Plugs Yourself?

Yes, spark plug replacement is often a do-it-yourself job. Most vehicles have easy enough access where even a beginner can change their own plugs. Depending on your experience level, it may take you anywhere from 20 minutes to a couple hours to do the job.

Before you attempt DIY spark plug replacement, validate that the spark plugs need to be replaced before you spend money on new spark plugs that you do not need, and throw away spark plugs that you could have gotten more use out of.

One benefit that you lose with replacing the plugs yourself is that when a professional mechanic inspects your spark plugs, they may have other recommendations based on their condition. The way the electrodes look or if there’s any oil on the spark plugs can indicate other car issues that need to be resolved or your new plugs will simply get fouled up again.

Preventative Maintenance

For the long term health of your car’s engine, follow the manufacturer’s recommended maintenance schedule. If the manufacturer recommends replacing spark plugs at a certain mileage, be sure to schedule an appointment with a trusted mechanic to replace them (or do it yourself).

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