Bad Spark Plug Symptoms: How To Tell if Your Plugs Are Bad

You could have all the premium fuel in your gas tank, the most technologically advanced engine under the hood, and some of the most cutting-edge technology you could ever want in your car.

But if your spark plugs are bad, it’s going to wreak havoc with your motor’s combustion process.

Who thought something as small and cheap would play such an important in the proper functioning of your engine?

Spark plugs are the reason why the air and fuel mixture in your engine cylinders combusts, kickstarting a series of events that make your wheels turn.

Needless to say, they need to be checked regularly and replaced when needed. That’s why many car manufacturers go out of their way to make it as easy as possible to replace them.

But how do you know when it’s time to do so?

In this guide, we’ll tell you all there is to know about the telltale signs of bad spark plugs so that you know exactly when to replace them.

Bad Spark Plugs: Visual Signs and Other Symptoms

Worn out spark plug

Most problems in your car can be diagnosed simply by keeping your eyes, ears, and nose open. Sure, you can always go to the mechanic, but being able to tell when something is up with your car will go a long way.

If you suspect that something is up with your spark plugs, the best thing you can do is inspect them visually. Doing this every 10,000 or 15,000 miles will go a long way.

Spotting bad-looking spark plugs is easy; all you need to do is unscrew them one by one and inspect them. The hard part is knowing what to look for.

Here’s a quick visual inspection list for your reference:

  • Clean greyish electrode, yellow to brown: Normal
  • Black soot or carbon deposits: Indicates incorrect air-fuel ratio, the spark plugs are probably colder than the application requires, or you have a clogged air filter.
  • Black sticky goop: Oil fouling. Oil is probably getting into the cylinders due to damaged pistons or piston rings. This is likely a serious that you should not ignore.
  • Yellow or brownish-yellow deposits on the insulator: Contaminated deposits of carbon or lead from fuel or oil additives.
  • Brown cinder-like deposits: Alloying additives in the form of residue from fuel additives. These will form an unburnable ash that might spill into the combustion chamber. Decarbonizing might be necessary.
  • Molten electrode with white blistered insulator tip: You’re probably using the wrong heat range.
  • Melting center electrode: Clear signs of detonation or pre-ignition.
  • Wet electrode, covered in fuel: Signs of a flooded engine.

Being more aware of your car’s needs will make you proactive, and push you to carry the right tools and spares in the car at all times, should the need arise when you least expect it.

Now that you know what to look for visually, here are some ways in which bad spark plugs will affect the drivability of your car.

Car Hesitates to Start

Person starting a car

Trouble starting your car could be an indicator of many different issues with your car, let alone bad spark plugs. But if this happens, it’s worth investigating your spark plugs first.

Worn-out spark plugs cause your engine to work harder because of the mismatch between spark timing and piston position.

This could lead to a long cranking time before your car starts. If your car doesn’t start at all, the battery could likely be the culprit, or it could just be the spark plug wires.

Low Fuel Economy

Empty fuel tank warning light

Bad spark plugs can easily damage your ignition coil which can snowball into a variety of problems like inefficient burning of fuel and misfiring.

The latter can cause the engine to send raw fuel into the exhaust stream which might also damage your catalytic converter.

Together, these issues lead to poor gas mileage and you might find yourself having to visit the gas station more frequently. When this happens, your check engine light will ideally turn on, letting you know something is wrong.

Inconsistent Idling

Tachometer showing inconsistent idling

As mentioned above, when the engine misfires, it sends unburned fuel into the exhaust system which sends mixed signals to the O2 sensor.

The O2 sensor will think your engine is running too rich which will cause your ECU to incorrectly regulate your fuel mixture, making it run too lean. This causes rough idling and might make the misfiring issue even worse.

Choppy Acceleration

Foot on gas pedal

Another problem caused due to your engine running lean and misfiring is choppy acceleration. You might notice that your engine pace loses momentum and then catches back up intermittently.

This is a classic case of bad spark plugs and fixing it is neither difficult nor expensive. Average spark plug replacement costs are extremely low. So if you are starting to notice the signs of bad spark plugs, you know exactly what to do.

Concluding Summary

If you’ve never performed any DIY maintenance on your car but wish to do so, replacing your spark plugs is a great place to start. In general, most spark plugs last around 20,000 miles, but you’ll probably want to replace them much more often than that.

The more vigilant you are of any abnormalities in your car, and the more mechanically proficient you become even harder maintenance jobs will get easier.

What spark plugs do you use? Let us know by leaving a comment below!

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