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8 Signs Of Bad Spark Plugs (+4 FAQs)

April 18, 2022

Identifying the warning signs of bad spark plugs can help you prevent engine damage and avoid costly engine repairs.

But what symptoms point to a defective spark plug?
And how do you replace a damaged spark plug?

In this article, we’ll answer those questions and more, including how long spark plugs last and whether you should replace them on your own.

This Article Contains:

Let’s get started.

8 Warning Signs Of Bad Spark Plugs

Here are eight symptoms of bad spark plugs you should watch out for:

1. Hard-Starting Engine

If your vehicle struggles to start in cold weather or after sitting for a few hours, faulty spark plugs or bad spark plug wires may be the culprit.

When you’ve got a worn spark plug, it can’t produce the spark needed to start the combustion process. As a result, your vehicle may crank for a long time before starting, or it may have a shaky start.

Continued hard-starts can damage your engine’s ignition system and drain the car battery.

2. Rough Engine Idling

Ideally, your engine sounds smooth and steady, and its RPM stays consistent.

But if you’ve got a bad spark plug, you’re likely to experience rough idling. Specifically, you’ll hear rattling noises, feel excessive vibrations, and see abrupt surges/drops in the internal combustion engine’s RPM.

Rough engine idling can happen when your ECU (Electronic Control Unit) tries to compensate for a failing spark plug and loss of power.

3. Poor Fuel Economy

A common symptom of bad spark plugs is a sudden drop in fuel economy (a.k.a. fuel efficiency or gas mileage).

That’s because a faulty spark plug can’t burn the air fuel mixture coming into the combustion chamber efficiently. Consequently, fuel economy drops as fuel consumption spikes, and you’ll have to fill up the gas tank more frequently.

4. Slow Acceleration

When you’ve got a malfunctioning or worn spark plug, you may notice that the car drives sluggishly or isn’t as responsive as before. Essentially, you’ll experience engine hesitation and feel like your car has lost its get-up-and-go ability.

Slow acceleration can also result from a bad fuel pump or dirty fuel injector.

However, since a bad spark plug is the most common culprit for engine hesitation, start by troubleshooting your spark plugs.

5. Engine Misfires

An engine misfire usually indicates that you’ve got a fouled spark plug.

Simply put, an engine misfire is when one or more cylinders in your engine don’t produce power. It happens due to incomplete combustion of the air fuel mixture in the cylinder and can sap the engine power.

If your engine misfires, you’ll notice a sputtering sound coming from the car engine, violent shaking, or a sudden drop in engine power.

6. Engine Knocking

Sometimes, you’ll hear a loud knocking sound coming from the engine.

Engine knocking results from the pressure shock waves created due to uneven burning of fuel in the combustion chamber.

If left unaddressed, engine knocking can cause irreversible damage to your car’s piston head, piston compression rings, cylinder head, cylinder head valves, and other critical engine components.

7. Check Engine Light Illuminates

Your car’s Check Engine Light may turn on when there’s a spark plug problem.

That usually happens when you’ve got a failing spark plug slathered in oil or running too hot, causing issues with the engine operation.

However, the Check Engine Light can also activate if there are issues with your spark plug wire, coil pack, catalytic converter, etc.

8. Exhaust Smells Like Gas

If your spark plug isn’t burning the airfuel mixture properly, unburned fuel (or gasoline) will make it to the car’s exhaust system. As a result, you may notice that the car’s exhaust emissions smell like gasoline.

While different causes can lead to the smell, the spark plug is a good starting point for diagnosis.

Next, we’ll answer some FAQs related to replacing your car’s spark plug.

4 FAQs About Replacing Faulty Spark Plugs

Here are answers to four common questions car owners ask about replacing a bad spark plug:

1. How Much Does Spark Plug Replacement Cost?

Replacing your vehicle’s spark plugs can cost around $100-$250 on the low-end and about $250-$500 on the high-end.

This cost includes:

The total cost can also vary depending on the make and model of the vehicle, the brand of platinum or copper plugs used, your location, and whether you take the car to a dealership or an auto repair shop.

2. How Long Does A Spark Plug Last?

Your spark plug’s lifespan can vary greatly depending on the type of spark plugs used (for example, a copper spark plug or a platinum spark plug), your driving conditions, etc.

However, manufacturers generally recommend changing the spark plug every 30,000 miles.

Beyond that limit, your spark plugs will start to:

That being said, if your vehicle uses high-end iridium or double platinum spark plugs, they can last for around 100,000 miles.

Check your car owner’s manual for a more accurate estimate of the spark plug’s lifespan.

3. Can I Replace A Faulty Spark Plug On My Own?

You can.

But do that only if you’ve got ample automotive knowledge/experience. 

If you replace the spark plugs the wrong way, you may hurt engine performance and lead to more expensive repairs down the lane.

Besides, with a professional mechanic, you can get better recommendations based on the spark plug’s condition.

For example, if your spark plug has oil deposits, other issues may exist with the car engine and ignition system. When that’s the case, even if you were to install new spark plugs, they’ll simply get fouled again.

If you’re looking for a hassle-free way to replace your car’s spark plugs, contact RepairSmith

RepairSmith offers a range of mobile car care and repair services at upfront and competitive prices.

4. How Is A Defective Spark Plug Replaced?

First, your mechanic will gather all the necessary tools required for spark plug replacement — a spark plug socket, a ratchet, a spark plug gap gauge, a torque wrench, etc.

The mechanic will then:

Final Thoughts

Spark plugs are designed to last for a long time, but they sometimes fail prematurely.

If you notice any of the common signs of bad spark plugs mentioned above, get new spark plugs installed ASAP. Otherwise, your spark plug problem can increase fuel consumption, causing a drop in fuel efficiency, and lead to irreversible engine damage.

While replacing a fouled spark plug can be a DIY project, don’t do it yourself if you’re inexperienced.

For affordable and convenient spark plug replacement, get in touch with RepairSmith.

Our ASE-certified technicians will come to your driveway for spark plug replacement, and all your auto repair and maintenance needs.