Look Out for These 9 Different Symptoms of Bad Spark Plugs!

Spark plugs can be a frustrating component to deal with when they go wrong. Identifying them as the sole cause of a problem can be difficult. As they are not replaced as frequently as they used to be, many drivers are likelier to notice issues like acceleration or misfiring problems. Therefore wanting to find and sort the issue.

However, before reading more into the symptoms of faulty spark plugs in this article, it’s important to note that they can develop problems similar to other engine parts. Therefore, diagnosing if they are the root cause of an issue can be challenging. Nevertheless, replacing spark plugs is a relatively inexpensive maintenance task that can benefit the engine’s performance, regardless of whether the plugs are defective or not.

coil packs

9 Common Symptoms of Faulty Spark Plugs

The issue with some of the problems below is that they have the same symptoms but are entirely different faults. The only way to be sure the spark plugs aren’t working correctly is to remove and inspect them for fouling, rust, or an improper gap. But you need to be sure you’re even starting in the right place, so you should be aware of the potential symptoms. 

Lack of Performance When Accelerating

The poor performance is because bad spark plugs are not igniting and burning the air-fuel mixture injected into the engine. Without fuel being burnt, the car doesn’t have any go. Usually, poor performance is the first symptom noticed when a car has bad spark plugs.

Engine Hesitation Under Acceleration

Although engine hesitation and lack of performance when accelerating are very similar, the thing to be aware of is the engine hesitating or stuttering. The spark plugs fire in a specific sequence. However, with a faulty spark plug, there can be a delay in the ignition sequence, causing the engine to hold back when accelerating. 

Poor Fuel Economy

Poor fuel economy is a classic symptom of faulty spark plugs; when the spark plugs fail to burn all the fuel injected into the cylinder, usually because they are dirty or fouled, the unburnt fuel is still forced out through the exhaust. As a result, the car has limited power. You press harder on the gas pedal, expecting the vehicle to accelerate, but because the fuel isn’t burnt, the performance suffers, and the engine tries to keep up by injecting more fuel. This isn’t drastic, but you may be topping up the gas more often than usual with bad spark plugs.

Engine Management Light On

When the engine management light is on (check engine light), and you connect the car to a code reader, it won’t specifically say the spark plugs are bad. But, one of the many engine sensors will display a fault when they are not to blame; it is tricky to diagnose because you rarely find the spark plugs that cause the E.M.L. to illuminate.

Engine Misfire

Engine misfires from faulty spark plugs will not be as prominent as a misfire is typically described; they are usually minor. Expect to hear and feel a rough-running engine initially because one or more spark plugs are not firing correctly. The misfire may only be noticeable as a rough idle and vibrations resonating through the car, nowhere near as harsh as with a faulty coil pack.

Loud Engine

If the engine noise has increased, the spark plugs may be fouled (covered in oil); you can expect fluctuating R.P.M.s and excessive engine noise because the spark is not strong enough to ignite the fuel correctly. 

Struggle Starting the Car

Depending on how many spark plugs are bad, they may struggle to get the car going. The air-fuel mixture injected in the engine needs igniting for the engine to run; with faulty spark plugs, the spark may not be strong enough, if they even spark at all. Usually, the car will fire up eventually but will run rough once it is started.

Strong Fuel Smell

The pungent smell comes because the fuel is unburnt in the engine and is forced into the exhaust. If not hot enough, it is passed out of the exhaust tailpipes so the smell of unburnt fuel can be smelt in the cabin and from the back of the car.

Black Smoke Coming From the Exhaust

Excessive black smoke from the exhaust indicates that unburnt fuel is burnt in the exhaust instead of the engine cylinder. This is because the spark plugs aren’t igniting the fuel; instead, it is forced into the exhaust, which, when hot enough, will ignite the fuel and give off black smoke. This can be damaging to the catalytic converter.

old spark plug

Can You Drive With Bad Spark Plugs?

Driving with bad spark plugs depends on the symptoms you experience; if you find the engine has developed a misfire, it will be impossible to drive. But, if you discover you are just experiencing a higher consumption of fuel, then, for now, that is all you will experience. The problem comes when your car develops other issues, and there is no telling how long that will take. 

If you are mechanically minded and prepared to remove spark plugs, remove the plug, give the tip a clean-off, check they are not damaged, and refitting may help you squeeze that extra few miles. However, by the time you’ve removed them, you may as well replace them if you have new ones to hand. Spark plugs aren’t particularly expensive. 

Can Bad Spark Plugs Damage Other Components?

Faulty spark plugs can damage other components in your engine. If the spark plugs are not functioning correctly, it can cause the engine to misfire, creating an unbalanced load on the crankshaft and damaging the bearings. Additionally, if the spark plugs are fouled or not firing correctly, excess fuel can enter the catalytic converter, which can cause damage over time.

How Often Do Spark Plugs Need Replacing?

Previously, spark plugs were replaced at annual service, approximately every 19,000 kilometers (12,000 miles), although spark plugs could go much further. Now, manufacturers do not replace spark plugs during every service; for most, the first replacement is when the vehicle is at 160,000 kilometers (100,000 miles). It is good to practice replacing them sooner as, for most average drivers, 160,000 kilometers is 10+ years. When spark plugs age, the material from the center loses its sharp edges, and ground electrodes are slowly worn away.

Replacing spark plugs every five years or 80,000 kilometers (50,000 miles) will eliminate any minor issues that develop with aging spark plugs. Some problems that can develop render the car undrivable, and with changing spark plugs being inexpensive, it’s not worth pushing them to the point of failure. 

Frequently Asked Questions

What Do Bad Spark Plugs Sound Like?

Bad spark plugs sound and feel like the engine is mildly misfiring. Suppose you haven’t experienced a car misfiring before. In that case, the engine is running extremely rough and not firing correctly, so the engine shakes around a lot, and you can hear and feel that when trying to accelerate.

Do Spark Plugs Affect Acceleration?

Spark plugs massively affect a car’s acceleration; without a good spark (produced by the spark plug), there isn’t anything to ignite the air-fuel mixture forced into the engine.

Do New Spark Plugs Make A Difference?

It’s difficult to tell that your spark plugs are operating at 100%. Generally, new spark plugs will improve the smooth running of an engine and restore any lost performance, especially if they have covered more than 30,000 kilometers. They won’t be bad after 30,000 kilometers, but the internals of spark plugs erode with age and don’t deliver the same quality of spark that new plugs give.

Final Thoughts

To avoid troubles, replace the spark plugs every 50,000 miles or five years. Spark plugs are now rated for 100,000 miles; however, it makes more sense to replace them sooner because everybody’s driving habits are different, and the spark plugs may go bad sooner. The most common symptoms of bad spark plugs are a rough-running engine, vibrations felt through the car, and a noisy engine. If you suspect an issue, you can take the spark plugs out and inspect them or have a mechanic check the vehicle for you.

My name is Tom although my friends call me Tommy. Messing around with cars and bikes has always been a hobby of mine even from a young age. So I made it my day job 17 years ago. I am a fully qualified mechanic as you would expect. I've worked in all different areas of the motor trade, valeting, panel beating, engine repairs, I'm sure you get the idea. I enjoy sharing my wealth of knowledge and experience with others, which is the reason I spend a lot of time here writing for this website.

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