Symptoms of bad spark plugs – 9 different problems to look out for!

Spark plugs are pesky when they go wrong, as they are difficult to pin down as the sole cause of an issue. Because they aren’t replaced as often as they used to be, many more drivers notice problems when driving, which can be traced back to faulty spark plugs.

Before you read more about the symptoms of bad spark plugs, it is essential to note that spark plugs develop problems similar to other components; if you are experiencing issues taking your car to your local mechanic to inspect will rule out the spark plugs or another fault altogether.

9 different symptoms of bad spark plugs

The issue with some of the problems below is there are the same symptoms as completely different faults. The only way to be sure the spark plugs aren’t working correctly is to remove and inspect them for any fouling, rust, or an improper gap.

1. 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.

2. 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 sequence, but with a faulty spark plug, there can be a delay in the ignition sequence and the engine surging. Engine surging is more air being forced into the engine than is typically needed. An improper burn air-fuel ratio when burnt causes a jerky engine when accelerating.

3. Poor fuel economy

Poor fuel economy is a classic symptom; when the spark plugs fail to burn all the fuel injected into the cylinder because they are dirty or fouled, the unburnt fuel is still forced out through the exhaust, and the car doesn’t develop any propulsion. 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 feel that you’re topping up the gas more often than usual.

4. Engine management light illuminated

The engine management light on and connected to a code reader won’t specifically say spark plugs are bad. But, one of the many engine sensors will display a fault when they are not to blame; a bit tricky to diagnose because you rarely find the spark plugs that cause the EML to illuminate.

5. 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 at first because one or more of the spark plugs are not firing correctly. They don’t usually stop working; 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.

coil packs

6. Loud engine

In a loud engine, the spark plugs are usually fouled; you can expect fluctuating RPMs and excessive engine noise. If the spark plugs fail to ignite the air-fuel mix, the engine misfires, causing rough running, vibrations, and a noisy engine.

7. Struggle starting

They may struggle to get the car going depending on how many spark plugs are bad. 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 spark at all. Usually, the car will fire up eventually but will run rough once started.

8. Strong fuel smell

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

9. 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 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 find 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 take spark plugs out, remove the plug, give it a clean-off, ensure 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.

How often do spark plugs need replacing?

Previously spark plugs were replaced at annual service, approximately every 12000 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 100,000 miles. It is good to practice replacing them sooner as, for most average drivers, 100,000 miles is 10+ years. When spark plugs age, the material from the center loses its sharp edges, and ground electrodes are slowly worn away.

Ideally, replacing spark plugs every five years or 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.


What does bad spark plugs sounds like?

Bad spark plugs sound and feels 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 20,000 miles. They probably won’t be bad after 20,000 miles, 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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