9 Symptoms of Bad Spark Plugs

Spark plugs play a crucial role in the combustion process. They receive the high voltage electrical current from the ignition coil. The spark plug is used to ignite the mixture of air and fuel in the combustion chamber and helps prevent serious issues with your car.

9 symptoms of bad spark plugs

This ignition occurs a thousand times a minute to keep your car moving while you are driving. If your spark plug is failing, then you may notice that your car is having issues starting or experiencing rough idles.

The list of warning signs can help you detect a faulty spark plug in your car and avoid major malfunctioning issues. Here are 9 symptoms of bad spark plugs and what happens when spark plugs go bad.

Key Takeaway

9 Symptoms of Bad Spark Plugs

  • Misfiring engine
  • Lower fuel economy
  • Lack of acceleration
  • Trouble starting your car
  • Rough engine idling
  • Compression leakage on the spark plug
  • Check engine light
  • High Mileage
  • Using the wrong heat range spark plugs

1. Misfiring engine

Increased engine misfiring is often a clear indication of a faulty spark plug. When dealing with misfiring issues, your engine won’t function properly. Exhaust and emissions will increase and as your engine power decreases, so will your fuel efficiency.

When the electrode of the spark plug is completely worn, the engine will start to misfire. This means that the air and fuel mixture will not get ignited during the power stroke. This will cause the engine to shake at all speeds. There is a good chance that the check engine light will turn on and trigger misfire codes P0300 through P0312.

Just one misfireOpens in a new tab.

2. Lower fuel economy

9 Symptoms of Bad Spark Plugs

Old spark plugs can drastically decrease your car’s fuel efficiency because of its inability to combust properly. When your spark plugs are old or worn out, the gap between plug electrodes can either increase or completely close. This can negatively impact your fuel economy along with the emission system in your car.

A strong spark is necessary to get the vehicle running efficiently. When a spark plug is worn, more gas will be used to get the vehicle running. A bad spark plug can decrease the fuel economy by 30%. Basically, the money that you are spending on gas due to bad fuel economyOpens in a new tab.

3. Lack of acceleration

9 Symptoms of Bad Spark Plugs

If your spark plug is damagedOpens in a new tab.

The lifespan of your spark plugs depends on multiple factors including condition and the type of spark plugs used in your car. Usually, all spark plugs go badOpens in a new tab.

4. Trouble starting your car

Faulty spark plugsOpens in a new tab.

The spark plugs are responsible for providing the spark to ignite the air and fuel mixture during the power stroke. When the spark plug electrode is severely worn, there would be a larger electrode gap. The sparks produced would be weaker due to the wider gap. The vehicle needs a good spark, compression, and air and fuel mixture in order to start.

In this case, a weak spark will make the starting difficult. Extremely bad spark plugsOpens in a new tab.

5. Rough engine idling

9 Symptoms of Bad Spark Plugs

Rough idling is a clear sign that there is an issue with your spark plugsOpens in a new tab.

Anytime you begin to feel vibrations throughout your vehicle, there’s a good chance your car is trying to tell you there is a serious problem with the spark plugs. You should remove one spark plug and perform a visual inspection. Check the spark plug and see if it’s contaminated with oil. Then, look at the electrode gap and measure it. If the gap is too big, change all of the spark plugs.

6. Compression leakage on the spark plug

Remove some of the spark plugs and perform a visual inspection. Look at the top insulator. If there is a discoloration on the top ceramic insulator, usually a brown and orange color, that means that there is a leakage from the spark plug and a new spark plug is needed.

The ceramic insulator runs the entire length of the spark plug and it is insulating the center electrode from the ground electrode. If the ceramic is cracked, the spark could jump out of the crack and it won’t travel to the electrode and ignite the air and fuel mixture. You can’t fix a spark plug with a cracked ceramic insulator and you will have to replace it.

Inspect the terminal nut and look for any rust. If the terminal nut is rusted, you can sand it down. Also, check the inside of the spark plug wire because there is a good chance that’s rusted too.

While you are holding the spark plug, inspect the electrode as well. If the gap is just too wide, you should replace the spark plugOpens in a new tab.

7. Check engine light

9 Symptoms of Bad Spark Plugs

Modern cars are equipped with a lot of sensors that will give you a lot of data if you have access to a scanner. The scanner that I use is an Autel OBD (check price on amazon.comOpens in a new tab.

When the engine misfires, the check engine light will come on. Usually, the trouble codes associated with misfire are P0300 through P0312. You will need a scan tool to be able to read these codes. If you are not in the mood of getting a scan tool, just think about this.

A car that runs with bad spark plugs will spend more fuel because the engine will have to compensate for the lack of power by throwing more fuel into the combustion chamber hoping it will ignite. That fuel usually ends up unburned and exits through the exhaust. All that money wasted on gas could get you a scan tool that costs around $80 and you will be able to diagnose the issue yourself.

8. High Mileage

9 Symptoms of Bad Spark Plugs

All car parts have a lifespan and will have to be changed eventually. The lifespan of the spark plugs is around two and a half years or between 20,000 and 30,000 miles. If you just got your vehicle and you are not sure if the previous owner changed the spark plugs, go ahead and change them as preventive maintenance. When you should change your spark plugs is listed in your owner manual.

If you owned the car for a long period of time and you drove more than 30,000 miles, as soon as you notice any of the previous warning signs of a failing spark plug, go ahead and replace the whole set of spark plugs. The number of spark plugs depends on how many cylinders your engine has. If you have a V8, your engine will have 8 spark plugs and a V6 will have 6 spark plugs.

9. Using wrong heat range spark plugs

Every engine comes with a spark plug that has a certain heat range. The spark plugs have to have a certain heat range because it needs to maintain a certain temperature to in order to burn off the carbon deposits that are going to get on it.

If the spark plugs are too cool, the carbon deposits will stay unburned on the spark plug and will decrease the ability of the spark plugs to create sparks. If the spark plugs are too hot it can cause excessive temperatures inside your combustion chamber. Basically, the spark plug will cause detonation or engine knock which untreated can do big damage to the engine.

Always check the owner manual to see which type of spark plugs are recommended for your engine and install those.

Igor Iwanowski

I am a certified Automotive Service Excellence (ASE) mechanic since 2018. I specialize in Brake systems, dashboard warning lights, EGRs, general engine problems, EVAP and Emissions issues.

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