Spark plugs are designed to do only one thing—generate powerful sparks inside the engine's chambers, igniting the air-and-fuel mixture to produce an explosion. That continuous process creates the power you need to move your vehicle forward.
Spark plugs typically last around two years. When they start failing, you'll notice symptoms related to the engine's performance. The engine will struggle to start and behave roughly even when the vehicle is idling, and there will be misfiring and surging. During acceleration, worn spark plugs will fail to deliver the expected power increase.
Spark plugs are hidden inside your engine, so you must understand the symptoms they’ll show when they begin failing. This guide will tell you everything you need to know to recognise failing spark plugs.
How Does a Spark Plug Work?
To better understand how your spark plugs fail, you must first know how they work.
A spark plug's function is all in its name. It receives high-voltage electricity at one end and generates a strong spark on the other. That spark is used to ignite the fuel-and-air mixture inside the engine's chambers to produce power.
Engines have several spark plugs, continuously generating high-voltage sparks while the engine is running.
Spark plugs are critical in your engine's ongoing combustion process. As you'll see in the next section, failing spark plugs will cause several symptoms, affecting the engine's ability to start and run smoothly.
How Do You Know If Your Spark Plugs Are Failing?
Standard spark plugs will last an average of 2 years before they start failing, but their lifespans can be shortened by other engine issues exposing the plugs to excessive wear and tear.
Failing spark plugs will undermine your engine's performance and cause unstable driving, so it’s always best to replace your plugs when they start going bad.
Whether your spark plugs begin failing due to old age or other forms of damage, they will show some or all of these symptoms:
#1 Starting Problems
A combustion engine needs three things to function—air, fuel, and sparks. You guessed it, spark plugs are responsible for the third part of that equation.
When your spark plugs start to fail, they'll struggle to produce sparks, if at all, causing difficulty in starting the engine when you turn the ignition key.
Your engine might take longer than usual to start, or it’ll continue cranking without ever starting. Not only does the spark plug fail to kickstart the combustion process, but it also fails to keep it going.
You’ll know that’s the case when you troubleshoot your car and find that your battery, alternator, and starter are working correctly. When that happens, the spark plugs are the most likely culprit.
#2 Misfiring and Surging Engine
Even if your spark plugs manage to start the engine, you'll likely experience unstable combustion along the way. From the driver's seat, the symptoms you'll feel are engine misfiring and surging, which might happen continuously or intermittently.
Misfiring happens when one or more engine cylinders don't produce power. In other words, the spark plugs in those particular cylinders fail to produce any sparks, even though the others do.
Meanwhile, surging is when the vehicle suddenly jerks forward. That happens because your failing spark plugs do not ignite the compressed air-and-fuel mixture at the precise moment. As a result, the engine's normally perfect sequence of combustions is disrupted.
#3 Weak Acceleration
Failing spark plugs will also struggle to keep up when you increase your demand on the engine. In other words, your car can't respond accordingly even when you press down hard on the accelerator.
Pressing down on the pedal sends more air and fuel into the engine. However, failing spark plugs struggle to produce sparks at a faster pace.
Due to that, you’ll experience weak acceleration. After that, you’ll likely experience misfiring and surging (as described earlier) if the spark plugs start to catch up and suddenly produce strong sparks.
#4 Rough Idling
It's not only when you're starting or accelerating that failing spark plugs become noticeable. Even when you place no demand on your engine, i.e. while idling, you'll find that your engine fails to do so smoothly.
The engine's RPM readings will surge up or drop down without apparent reason, despite your car being parked and in neutral.
That’s due to spark plugs failing, which disrupts the engine’s smooth combustion process and affecting the engine’s performance even when standing still and just trying to stay running.
#5 Excessive Fuel Burning
Last but not least, failing spark plugs will also demonstrate subtle symptoms like excessive fuel burning. Of course, you’ll only notice after some time—your fuel tank empties sooner than usual, and you find yourself at the petrol station more often.
When a spark plug doesn't generate sparks reliably, it disrupts the engine's overall efficiency. As time passes, the engine draws in more fuel than usual to ensure it can produce the power you expect.
What Should I Do with a Bad Spark Plug?
There’s only one thing you can do with failing spark plugs: replace them with new ones. Even if only one or two are failing, it’s always best to replace them simultaneously. That way, you can rest assured that they’ll last roughly the same lifespan and fail around the same time in the future.
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By Ray Hasbollah