6 Signs that Your Spark Plugs May Be Going Bad

Jan 09, 2020


Spark plugs play a vital role in a well-running engine. If your spark plugs are old or damaged, then they should be replaced as soon as possible.image 1

What do spark plugs do? As the name implies, they provide the (literal) spark that powers your car's engine. The science behind it works like this:

  • Every internal combustion engine has multiple chambers, or cylinders.
  • In each cylinder, air and fuel are mixed together and then compressed into a tiny space by a piston.
  • At that point of maximum compression, the spark plug explodes the air/fuel mixture.
  • In turn, the explosion drives the piston downwards.
  • The downward motion of the piston serves as the mechanical power that keeps the engine running.

Worn out spark plugs cause misfires, or explosions in one cylinder that are out of sync with all of the other cylinders. Misfiring or non-functioning spark plugs can cause serious performance issues in your vehicle. Here are 6 signs that you may need to have your spark plugs replaced:

Image 21. Your Check Engine Light Comes On

Misfiring spark plugs will often trigger a check engine light on your dashboard. In some vehicles, a flashing check engine light indicates that your engine is suffering from catastrophic misfires. In other words, you need to seek professional assistance right away!

2. Your Car Idles Roughly

If your car engine is on and your transmission is in park, a bad spark plug will often cause your car to idle very rough. You'll feel a distinct vibration from the engine, or hear a knocking or rattling noise coming from under the hood.

3. Your Car Misfires While You are Driving

A misfiring car is a clear indicator that your spark plugs are going bad. You'll experience erratic, jerking motions as you drive your vehicle, often accompanied by loud knocking sounds.

These symptoms occur because your car's engine is operating with significantly reduced power. For example, the loss of one spark plug in a 4-cylinder engine means that the three good cylinders need to generate enough force to compensate for a 25% (1/4) loss in power.

4. You Have Trouble Accelerating

Misfiring spark plugs can also lead to reduced accelerating power. Since your vehicle relies on the precise synchronisation of its multiple pistons to smoothly accelerate, the loss of even one spark plug can turn normal acceleration into a power-sucking strain on your engine.

5. You Can't Start Your Carimage 3

There are many potential reasons as to why a car won't start. Misfiring or worn out spark plugs are one factor that should be evaluated if more common causes (such as a dead battery or a worn out alternator) have already been discounted. If you are facing this situation, then you should seek a professional diagnosis from a qualified mechanic.

6. Your Car is Losing Fuel Economy

Even if your car is still functional with a misfiring spark plug, you'll likely see a marked dip in your vehicle's ratio of litres to 100 kilometres. In fact, studies have shown that bad spark plugs can cause a decrease in fuel efficiency of up to 30%!

Please note that the 6 signs discussed above do not form a comprehensive list of all the issues that could occur from a damaged or failing spark plug. There are several more performance issues that could have their root in misfiring spark plugs, or bad spark plug wires.

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If you have experienced any of these problems with your vehicle, Mr. Lube offers spark plug inspection and service, no appointment needed. If the inspection turns up an issue, our qualified technicians will replace any bad spark plugs, and get you back on the road in short order.