What Symptoms Indicate A Bad Spark Plug?


If your spark plugs aren't doing their job right, there are some obvious and not-so-obvious signs you should look out for.

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While we may use them every day and think nothing of it, our vehicles are advanced machines with lots of intricate parts working together to give us that seamless experience. However, high levels of complexity mean a higher possibility for malfunction. The engine, in particular, relies on a number of separate systems to function, and some of the smallest and often overlooked components, such as spark plugs, are vital. These electrical doodads are essential for your engine and car to start up and run, so if something goes wrong with them, trouble is sure to follow. Luckily, the symptoms of a bad spark plug aren't too hard to spot.

What Is a Spark Plug, And What Does It Do?

The spark plugs in a car engine are responsible for the spark that ignites the air and fuel mixture, which initiates the combustion process that creates power in the engine. They are located at the top of a cylinder head in the combustion chamber, and generally speaking, you will have a spark plug for each cylinder - so four plugs in a four-cylinder engine, six plugs in a V6 like a Dodge Charger. In some cases, though, you will find two spark plugs for every cylinder. In twin-spark or dual-ignition V8 engines like those on the Jeep Grand Cherokee, you'll find a total of 16 plugs.

Spark plug life spans vary depending on the brand and what materials they are made of. The interval for when you should change spark plugs will be indicated by the manufacturer of your vehicle, so be sure to check your owner's manual for how often to change these, and follow our basic maintenance schedule for a handy guide if you're unsure.

Changing Spark Plug BMW

Symptoms of Faulty Spark Plugs

A faulty spark plug can have a serious impact on the running of your vehicle, and it may even lead to decreased performance. It can also negatively affect things like fuel economy or even damage your engine, leading to far more expensive repairs. To help you avoid this, here are the symptoms of a bad spark plug to keep an eye out for:

  • The car struggles to start: This is a very obvious sign. Ignition coils provide the charge for the spark plugs, and when an ignition coil fails, the spark plug won't be able to start the combustion process. Sometimes the spark plugs themselves are the issue, being worn down or clogged and not able to produce that essential spark.
  • The engine misfires: Since there are multiple possible causes for a misfire, this is a less clear-cut symptom. However, a faulty spark plug can certainly cause a misfire. You may even hear a popping sound or notice that the engine suddenly loses power for a second before catching again.
  • Weakening engine performance: A knock-on effect of misfires, acceleration and overall performance will suffer if any of the cylinders fail to ignite its fuel mixture. This is a telltale sign of spark plug failure.
  • Rough idling and loud engine noises: When a spark plug isn't working, the corresponding cylinder on the engine fails to function properly, too. When it's pumping but not actually getting anything done, it can rattle loudly.
  • Fuel efficiency: A slightly less obvious sign that a spark plug is bad would be if your overall fuel economy takes a hit. You'd need to be the type of person that pays close attention to your driving and regassing, but you will notice a decrease in efficiency since at least some of the fuel isn't being ignited and burned properly.
  • Poor emissions test results and exhaust fumes smell like gas. The latter part of this problem is eminently noticeable. Since some of the fuel isn't being ignited in the engine, it makes its way into the exhaust system, where it evaporates and creates a strong odor. Performing an emissions test will show that there is a lot more gasoline vapor than there should be.
Bad Spark Plug Symptoms unsplash.com

How To Change Spark Plugs

Once you've established what the problem is, you'll want to know how to change spark plugs and the wires associated with the system. The tools needed to change spark plugs generally include:

  • New spark plugs
  • Spark plug socket
  • Some wrenches and a ratchet
  • Some engines may require more specialized tools such as a universal joint extension.

Here are the steps to follow when you're changing out a spark plug:

  1. Ensure that the engine is cool. To be on the safe side, you could disconnect the battery by following these steps.
  2. Clean any dirt from around the area you'll be working in - compressed air does a great job here.
  3. Remove the spark plug wires, being careful not to damage the rubber boot or the wires themselves. Avoid pulling on the wires, and make sure to lift from the boot. Clear out the spark plug well with more compressed air once the boot has been pulled out.
  4. Using a spark plug socket (connected to an extension, where needed) and a ratchet, push down onto the plug and break it loose.
  5. You should be able to screw the plug out by hand once it's broken loose and remove it. It's a good idea to inspect the old plug and compare the new plugs to ensure they are the correct size.
  6. Check the plug gap on new spark plugs (see below) before installing them.
  7. Use the socket to install and tighten the new plugs - be careful of over-torquing. Hand-tightening each plug is ideal, using a torque wrench for the last bit.
  8. If you're not replacing the spark plug wires, you'd simply push them into the relevant well until it clicks into place.
  9. To replace the wires, ensure you have the correct wire size or length for the appropriate plug.
  10. Once you've done all the plugs and connections, start the car to check everything is working well.

The total replacement time for spark plugs is around one hour for a four-cylinder engine. If you want to know when and how often you should replace car spark plugs and wires, consult your owner's manual. However, most mechanics suggest installing new spark plugs every 20,000 to 40,000 miles, unless something goes wrong before then.

Replace Spark Plugs Pexels.com

How to Gap Spark Plugs

As the name implies, 'gapping' refers to the space between the ground electrode and the tip of the center electrode. The size of this gap is essential for optimal functioning - if it's too wide, you may get an inconsistent spark or no spark at all. If it's too small, you could have weak sparking and poor fuel ignition.

Most spark plugs sold in the USA are pre-gapped, but it's worth checking as these may be damaged during shipping. The gap on your spark plugs should be matched to your vehicle and engine specifications, as there isn't a universal or specific rule for all types of plugs. You can find the correct gap spec either in your owner's manual or on a label in the engine bay. To check, you can use various gapping tools, including a feeler gauge, a coin-style gauge, or a wire gapping tool that helps you identify if you need to open or close the gap to within spec. Just be careful not to damage the platinum or iridium tips during the measuring process, and make any adjustments in small increments.


What happens if you don’t change faulty spark plugs?

In extreme cases, your engine will no longer be able to start. However, the most likely outcomes are decreased performance, poor fuel economy, and rattling or unsettling noises while driving. Misfires are also common.

Do you need to change all your spark plugs at the same time?

If you're going to replace one, you may as well do them all. This way, they all have the same lifespan and can be more easily swapped out when it comes time for maintenance and replacements. It also ensures more consistent performance when driving.

How much does it cost to change spark plugs?

There are different types of plugs ranging from copper tip platinum and iridium, and these increase in price, with copper being the cheapest. Prices can also vary between brands. However, you are most likely to pay between $66 and $100 for a set of spark plugs for a four-cylinder engine. If a mechanic does the work for you, then you should factor in an extra $100 - $150 for labor.

Do you have to use dielectric grease and anti-seize when you replace spark plugs?

Anti-seize is a lubricant that helps to prevent the threads from getting stuck in the engine when you're trying to remove the plug. Most modern plugs don't require this, so be sure to check your owner's manual and with the plug manufacturer - and, if you are going to use it, use it sparingly, only on the threads, and not near the electrodes where globs of it could interfere. Dielectric grease is an insulator used on the ceramic part of the plug to prevent moisture from corroding the plug and is a good - but not essential - product to use.

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