How to Replace a Bad Spark Plug?

For a spark plug replacement, you may need to remove the intake manifold and other components like the ignition coil or spark plug wire. Accessing and removing these parts can be difficult on some vehicles.

Moreover, a bad spark plug replacement job can lead to expensive engine repairs. That’s why it’s best to let a professional mechanic change your vehicle‘s spark plugs.

But if you still wish to know how it’s done, here’s a general guide on spark plug replacement: 

For the replacement, a mechanic will need:

• Fresh spark plugs
• Spark plug socket
• Ratchet and torque wrench
• Spark plug gauge
• Anti-seize lube
• Dielectric grease, etc.

Here’s the process they’ll follow:

1. Remove the engine cover.

2. Disconnect the ignition coil connector or the spark plug wire.

3. Remove the old spark plug using a spark plug socket.

4. Clean the spark plug hole to remove debris that can damage the cylinder head threads.

5. “Gap” the new spark plug using a spark plug gauge.

6. Apply a thin layer of dielectric grease to the spark plug boot.

7. Install the new spark plug.

8. Tighten the new plug with a torque wrench according to the manufacturer’s specifications.

9. Reinstall the ignition coil connector and engine cover.

Start the engine and ensure everything works after installing the new plug.

How Much Does Spark Plug Replacement Cost?

Replacing spark plugs can cost between $100 and $500:

• Spark plug: 
  • Copper spark plugs: $2 to $10 per piece
  • Iridium spark plugs:  $20 to $100 per piece
• Labor charges: $40 to $350

If you own an older vehicle with a distributor ignition system instead of a coil pack or coil-over-plug system, the spark plug wires may also require replacement:

• Spark plug wires: $90-$166
• Labor charges: $66-$170

The cost of replacing your spark plugs will also vary depending on the make and model of your vehicle, your location, and the spark plug brand.

How Often Will I Need to Replace Spark Plugs?

Most manufacturers recommend changing spark plugs every 30,000 miles. However, your spark plug’s lifespan can range between 10,000 miles and 150,000 miles depending on the:

• Spark plug metal type
• Spark plug manufacturer
• Spark plug design
• Vehicle’s make and model

Consult the owner’s manual on when to change old spark plugs and about the correct spark plugs for your vehicle. It’s a good idea to get the ignition coil and spark plug wires checked when your car gets a spark plug change. You may also get them inspected while going for other maintenance activities, like an oil change service.

But what if they go bad?
You should replace faulty spark plugs immediately. A faulty spark plug can result in an engine misfire. Frequent misfires could also lead to a damaged catalytic converter, reduced engine performance and fuel efficiency, and irreversible engine damage.

You may also end up with a dead battery from repeatedly trying to crank your car that won’t start due to spark plug problems.

What Are the Common Symptoms of Failing Spark Plugs?

Faulty spark plugs can lead to symptoms like:

1. Check Engine Light Turns On

A worn spark plug can cause your Check Engine Light (CEL) to turn on or keep flashing.

Why?
Spark plugs fouled with engine oil, fuel, or carbon won’t ignite the air-fuel mixture correctly. This can cause engine misfires and activate the CEL.

However, the Check Engine light can also appear due to spark plug wire or coil pack issues (other than a faulty spark plug).

2. Fuel Efficiency Drops

Bad spark plugs increase your vehicle’s fuel consumption because they lead to inefficient combustion. In fact, spark plug failure can reduce your car’s fuel efficiency (a.k.a. fuel economy or gas mileage) by around 30%.

Plus, unburned fuel from incomplete combustion will be expelled from the engine and may ignite in the catalytic converter, damaging it.

3. Engine Idles Rough

Under normal conditions, the engine should idle smoothly. But if your vehicle has a faulty spark plug, you may hear pinging, rattling, or knocking sounds due to misfires. Ignoring these signs of a bad spark plug can result in engine damage over time.

4. Car Struggles to Start

Worn spark plugs will struggle to fire your engine, likely stalling it. However, the starting troubles could also be due to a battery problem.

5. Engine Hesitates During Acceleration

An old spark plug can cause reduced engine performance. You could step on the accelerator pedal, but your vehicle won’t respond. Instead, it’ll accelerate sluggishly.

6. Exhaust Emissions Smell like Gasoline

Unburned fuel reaches the vehicle’s exhaust system when a worn spark plug doesn’t ignite the air-fuel mixture properly in the combustion chamber. Consequently, your vehicle’s tailpipe emissions may smell like gasoline.

The smell may also result from a clogged air filter, a leaky fuel injector, or a defective mass airflow sensor.

2 Spark Plug Replacement FAQs

Let’s explore some more about spark plug replacement through these two commonly asked questions:

1. Why Do Old Spark Plugs Need Replacing?

Every time a spark forms between the spark plug’s center and ground electrodes, it burns off small amounts of metal from each electrode. This gap increases as your vehicle‘s spark plugs constantly fire during engine operation.

With time, the spark plug gap grows so much that the required spark no longer forms, and the spark plug fails.

2. Do I Need to Replace All the Spark Plugs?

Ideally, you should change all the spark plugs in one go. Changing spark plugs helps you prevent situations like an engine misfire, bad gas mileage, etc.

If you’re replacing only bad spark plugs, each spark plug in your engine will have a different replacement cycle. Consequently, you’ll need to spend more effort and time with repairs whenever the spark plug on a specific cylinder fails.

Moreover, your engine works best when all the spark plugs have similar performance levels. For instance, if two spark plugs in your six-cylinder engine perform differently than the rest, it can cause the engine to go out of sync.