You’ve been driving and suddenly, something doesn’t feel right — maybe your engine shakes, or your car feels weak when you hit the gas. You might even hear strange sounds. If this sounds like your car, then the answer is simple: it could be bad spark plugs. Yes, those tiny parts can cause big problems. In this article, you’ll find out exactly how to tell if your spark plugs are going bad.
Engine Misfires and Rough Idling
Your engine is supposed to run smoothly. When it doesn’t, and instead starts shaking or making popping sounds, you might be dealing with a bad spark plug. A misfire happens when the spark plug fails to ignite the fuel mixture inside the cylinder at the right time.
This causes the engine to run unevenly. When one cylinder isn’t firing correctly, the others try to make up for it. This creates a shaky, jumpy feeling when your car is running — especially when you’re stopped at a red light or just starting the engine.
Bad spark plugs can cause the engine to shake and feel weak. You might even feel your steering wheel vibrating slightly when the car is idling. These signs are easy to notice, especially if your car usually runs quietly and smoothly. The shaking might feel small at first, but over time, it gets worse.
Another common sign is a weird popping or rattling noise from the engine. That’s the sound of the fuel not being burned correctly. You may also notice your check engine light turning on. Many drivers ignore it, thinking it’s just a small thing. But if it’s blinking or stays on, you should check your spark plugs right away.
Also, fuel that doesn’t burn completely because of a misfire can damage your catalytic converter. That part is very expensive to replace, and it helps keep your car’s emissions clean.
If your car shakes, misfires, or makes strange noises, don’t wait. Replacing spark plugs is a lot cheaper than replacing engine parts later.
- Engine shakes or runs uneven
- Popping or rattling noise
- Car feels rough when stopped
- Check engine light turns on
- Possible damage to catalytic converter
Trouble Starting Your Car
One morning you get into your car, turn the key, and nothing happens — or maybe the engine cranks for a long time before starting. This might not be the battery. In many cases, bad spark plugs are the real reason.
Spark plugs help ignite the air and fuel inside the engine to make the car start. If they’re worn out, dirty, or not firing properly, your engine will struggle. It might take longer to start or not start at all. This is one of the first signs that spark plugs are going bad.
Some people think this is just a cold weather issue, but even in warm temperatures, bad spark plugs can cause hard starts. It might feel like the battery is weak, or like the engine is trying but just can’t turn over. Sometimes it will start after a few tries — this is a big red flag.
The longer this goes on, the more damage can happen. Your engine will keep trying to fire up with poor spark, which wastes fuel and adds stress to your starter motor. Eventually, you could wear out the battery and starter too.
Also, keep in mind that one bad spark plug can cause this. It doesn’t have to be all of them. Just one weak plug can affect the whole engine.
To test this, pay attention to how often your car takes longer to start. If it’s becoming a pattern, get your spark plugs checked.
- Hard to start engine
- Long cranking before starting
- Feels like a weak battery
- Happens in all weather
- One bad plug can affect entire engine
Weak Acceleration and Sluggish Driving
Have you ever stepped on the gas and felt like your car isn’t moving as fast as it should? Or maybe it takes forever to reach a decent speed? This is another big clue that your spark plugs are bad.
When spark plugs don’t fire correctly, your engine doesn’t get the power it needs. Even if you press the gas pedal hard, your car might feel slow or unresponsive. It’s like your car is trying to move but doesn’t have the energy.
This issue shows up most when you’re trying to go up a hill, pass another car, or merge onto a highway. The engine might hesitate or feel like it’s “choking.” This makes driving not only uncomfortable but also unsafe. You need full power when you ask for it — and bad spark plugs take that away.
You may also notice that your car “jerks” or “bucks” when you try to accelerate. That’s another sign of uneven firing. It feels like the engine keeps starting and stopping power in small bursts.
Poor acceleration can make you feel like you’re driving a much older or weaker car. But often, this isn’t a big repair. New spark plugs can bring your car back to life.
Also, cars today rely on sensors to control the fuel-air mix. When spark plugs are not working well, it confuses the system, making the car run rich or lean — which can hurt both performance and fuel use.
- Slow acceleration
- Engine hesitates or feels lazy
- Jerking while speeding up
- Unsafe for merging or passing
- Can lead to poor fuel-air mix
Lower Gas Mileage
If your car is using more gas than usual, don’t blame rising fuel prices just yet. Bad spark plugs can make your fuel efficiency drop quickly.
When spark plugs don’t fire properly, not all the fuel gets burned. That means your engine uses more gas to do the same job. You might find yourself visiting the gas station more often than before — and wondering why.
A small drop in miles per gallon might go unnoticed at first. But over time, it adds up. You’ll spend more money every week, and your car might still feel weak. This is one of the sneakiest symptoms because it doesn’t feel like something is “wrong.” The car still drives — just not efficiently.
Most people don’t think about their spark plugs when trying to improve fuel economy. But keeping them in good shape is one of the easiest ways to save gas. Replacing them regularly can improve mileage by up to 30% in some cases.
Another sign related to fuel use is the smell of unburned gas. If you notice a strong fuel smell around the car or from the exhaust, it could be a sign that the plugs are not igniting fuel completely.
You can also track your gas mileage over a few weeks. If you notice a steady drop without any big changes in driving habits, your spark plugs could be the cause.
- Gas mileage drops
- Need to refill more often
- Fuel smell around car
- Waste of unburned fuel
- Replacing plugs can improve mileage
Engine Surging or Hesitating
One moment your car speeds up suddenly, and the next it slows down without you doing anything. That strange feeling of the engine “surging” or hesitating is another symptom of bad spark plugs.
This happens because the engine gets inconsistent sparks. Sometimes the spark is strong, other times it’s weak or missing. This causes the engine power to rise and fall suddenly, making the car feel jumpy. It’s especially noticeable when you’re driving at a steady speed. The car feels like it’s pushing and pulling without your input.
For beginners, this can be scary. You might think something major is wrong. But in many cases, it’s just the spark plugs misfiring.
Engine surging can also cause the RPM needle to jump around. You’ll see the needle move up and down even if you’re not pressing the gas. This kind of behavior can stress the engine and cause it to wear out faster.
It also makes the drive uncomfortable. It feels like the car is always changing speed, and it becomes hard to keep control — especially in traffic or on hills.
Dirty or worn spark plugs can also cause poor throttle response. You press the gas and the car doesn’t respond right away. This lag is another form of hesitation, and it can get worse if not fixed.
- Engine jumps or slows suddenly
- RPM needle moves oddly
- Poor throttle response
- Drive feels jerky or jumpy
- Caused by uneven spark strength
Strong Fuel Smell and Dark Smoke
Sometimes, bad spark plugs make it very clear — your car smells like fuel or the exhaust pipe spits out black smoke. Both of these are warning signs.
When spark plugs don’t work well, they let unburned fuel out of the engine. This fuel goes into the exhaust, where it may burn later or not at all. That creates a strong smell, almost like raw gas. You might smell it near the car when it’s running or even inside the cabin.
In some cases, you’ll also see dark or black smoke coming from the tailpipe. This smoke is a sign that too much fuel is being burned — or not burned correctly. It’s not just bad for the engine, but also for the environment.
Unburned fuel can also mix with oil and cause damage inside the engine. Over time, it can lead to dirty engine oil, clogged parts, and higher repair costs.
You should not ignore these signs. If your car smells strongly of gas, especially when idling or starting, it’s likely the spark plugs. Get them checked and replaced as soon as possible.
This issue can also cause your car to fail an emissions test. That’s a costly and stressful problem — especially if your state requires testing for registration.
- Strong fuel smell
- Black or dark exhaust smoke
- Unburned fuel escaping engine
- Bad for emissions and environment
- Can damage engine oil and parts
Final Thoughts
Spark plugs may be small, but their job is huge. They keep your engine running smooth, clean, and strong. When they go bad, your car starts showing signs — from shaky starts to poor gas mileage. If you notice any of these symptoms, don’t wait. A quick spark plug check can save you time, money, and stress. It’s one of the easiest and most affordable fixes you can do. Keep your car healthy by paying attention to how it feels, smells, and sounds.
| Symptom | Description |
|---|---|
| Engine Misfire or Rough Idle | Shaking, popping noises, and uneven engine sound while stopped |
| Trouble Starting | Long cranking, weak ignition, and failed starts |
| Weak Acceleration | Sluggish response when speeding up, especially during climbs or merging |
| Poor Gas Mileage | Frequent refueling, lower miles per gallon, fuel smell |
| Engine Surging or Hesitating | Sudden jumps or lags in speed while driving steadily |
| Fuel Smell and Smoke | Strong gas smell and dark smoke from exhaust pipe |
Frequently Asked Questions (FAQs)
Is it safe to drive with bad spark plugs?
No, driving with bad spark plugs is not safe. While your car might still move, the engine can misfire, shake, and lose power. This makes it hard to drive smoothly or respond quickly in traffic. Bad spark plugs can also damage other parts of the engine like the catalytic converter. The longer you wait, the more it may cost you in future repairs. It’s better to replace spark plugs early before it becomes a bigger issue.
Can bad spark plugs cause my car to shake?
Yes, bad spark plugs can make your car shake, especially when you’re idling or just started the engine. This happens because one or more cylinders aren’t firing correctly. The engine has to work harder to keep going, which causes it to run unevenly. You may also feel vibrations through the steering wheel or seat. This shaking is a clear sign that your spark plugs might need to be changed.
Do I need to change all spark plugs at once?
It’s a good idea to replace all spark plugs at the same time. Even if only one is bad, the others may be worn out too. Changing them all keeps the engine balanced and running smoothly. Mechanics also recommend replacing them together because it saves labor time and ensures all plugs are in the same condition. If you replace just one, the engine might still feel rough or unbalanced.
Is it expensive to replace spark plugs?
No, replacing spark plugs is one of the cheapest car repairs. The cost depends on the car type and plug quality, but it’s usually affordable. Most spark plugs cost between $2 to $15 each, and labor is usually under an hour. Even for newer cars with more expensive plugs, it’s still much cheaper than fixing engine damage caused by ignoring the issue. It’s a small price to keep your car running great.
Can bad spark plugs cause bad gas mileage?
Yes, they can. When spark plugs don’t work properly, fuel doesn’t burn fully. This means your engine uses more gas to make the same power. Over time, you’ll notice that your car needs more fuel to travel the same distance. You might also smell gas or see black smoke from the exhaust. Replacing old or dirty spark plugs can help you save money at the gas pump.
Do I need a mechanic to change spark plugs?
Not always. If you’re comfortable working on cars, you can change spark plugs yourself with the right tools. It’s a simple job on many vehicles. But for some cars, the plugs are hard to reach or require removing other parts. In that case, it’s better to let a mechanic handle it. Always check your owner’s manual before trying it on your own.
Is it normal to smell fuel when spark plugs are bad?
Yes, a strong fuel smell is a common sign of bad spark plugs. If the plugs don’t ignite the fuel correctly, unburned fuel escapes into the exhaust system. That’s what creates the gas smell. You might notice it near the engine or from the tailpipe. This isn’t just annoying — it means your engine is wasting fuel and possibly damaging the catalytic converter. Fixing the spark plugs usually solves the smell problem.
Can spark plugs affect engine sound?
Absolutely. Bad spark plugs can make the engine sound rough or uneven. Instead of a smooth humming noise, you might hear popping, ticking, or even knocking sounds. These noises happen because of misfires or poor ignition timing. If your engine sounds louder than normal or makes strange sounds, it’s a good idea to check the spark plugs. Quiet engines are happy engines — and bad plugs disrupt that peace.