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Changing Spark Plugs

Changing Spark Plugs

How do you know when you should change spark plugs?

By Alicja Gados

Is your car engine running rough, and is your car burning more fuel than normal? One of the first things you should do is check the spark plugs. Fouled, or old spark plugs can be the culprit.  You can easily check and replace them yourself or take them to a professional to diagnose.

To understand if and how to change spark plugs in your car, let’s first go over what they do and how to check if they need to be replaced.

What does a spark plug do?

How an engine works

Modern vehicles use a 4-stroke internal combustion engine.  For simplicity, to give you an idea of what the spark plugs do what you mainly need to know is that an engine does a four stroke cycle of tasks, separated by four distinct, separate stages.  These are the intake, compression, power and exhaust strokes.

The spark plug comes in on the power stroke, when the spark plug ignites the mixture of fuel and air, causing internal combustion that powers the engine.
Spark plug is what ignites the air-fuel mixture inside the combustion chamber.  This happens thousands of times per minute, and over the months and depending how much your engine is running, they wear out and need to be inspected regularly.  A typical plug lasts from 16,000-32,000 kilometers. 

The location of the spark plug in the cylinder. Image source: (

The spark ignites the fuel that causes an extremely fast burn in the cylinder.  The pressure makes the piston travel down very fast and turns the crankshaft, then transmission, and lastly - your wheels.

Understanding spark plugs

Gas powered cars run on what are essentially ‘explosions’ of energy.  The spark plugs channel the electrical current from the ignition, igniting the fuel.  It’s an essential component of any working combustion engine in all modern cars.

A spark jumps the ‘gap’ between two electrodes, it actually erodes, or burns off, minute amounts of metal from each electrode. Over time, the gap will become large so no longer can the spark make it through the gap, and your engine will misfire.  You’ll notice your engine is running rough, your fuel economy will go down, and if left unattended you may eventually get the “Check Engine” light. 
Better performance spark plugs

The spark plugs in good shape fire a hot spark that is blue in colour. A yellow or orange spark means weaker spark.

Most manufacturers now use extended life spark plugs to keep engines running at peak performance for longer intervals of time.  They do this by coating the electrodes with precious metals such as platinum, iridium, or yttrium that have higher melting points, and resist erosion longer.  However though it takes longer, these electrodes still do erode, and you will eventually need to replace the plugs.  Spark plugs with platinum or iridium tips can last upwards of 90,000 kilometers. 

If you can’t remember the last time you inspected your spark plugs, then it probably means you should check them.

A weak spark can signal that the gap between electrodes is too large. If the gap is too big it can cause a weak fire from ignition system.  Today, new spark plugs mean you don’t have to worry about the ‘gap’ and you can buy ready made spark plugs and just use as is.  However, if you like, you can use a gap gauge to make sure that the gap is the proper distance.  These gap gauges are available at auto parts stores. 

The gap is a fixed space. In older vehicles, and older plugs, sometimes this gap can be re-adjusted.  But this is not often done anymore.  

Do it yourself or take it to a shop?

Deciding whether or not to do this job by yourself really depends on the type of engine in your vehicle, and how easy it is to access the spark plugs.  For example, some 6 cylinder engines (V6 truck engines) have difficult procedures that require removing parts of the intake manifold.  Check your owners manual first to see what is involved.  If you’re not comfortable disassembling your engine to that extent, simply take in to a professional.

But if you have easy access to the plugs you can do it yourself. Simply purchase the spark plugs for your vehicle.  Checking spark plugs is simple and takes less than 10 minutes, and can be done using simple tools. 

How to check if you need new spark plugs

Checking and changing spark plugs is easier on old cars than on new cars. On old cars, spark plugs are readily accessible, but on newer cars, they tend to be under an engine cover, on the side or bottom where they are more difficult to access.

First of all, if your engine is running rough or your fuel economy is suffering, you can first check if fouled spark plug is causing it.  Quality spark plugs produce a better spark, increase fuel economy and performance of your engine.

Fouled plug

To see if you need new spark plugs, check to see if your current plugs are fouled.  A fouled plug happens when some sort of deposit, usually carbon, rests on the top of the plug and interferes with the ability of the plug to develop spark.  This is called a misfire and interferes with your fuel economy and performance.  If you have fouled plugs, replace them.

What you will need

You will need three things to change or check your spark plugs:

1.    A ratchet socket drive wrench
2.    An extension bar
3.    Spark plug socket, usually included in most ratchet socket sets

A ratchet is a wrench that clicks. For an auto repair tool kit, it is a must. You need a few of these ratchet wrenches and sockets that go with them.  The sockets are interchangeable and allow you to fit almost any sized bolt, to loosen or tighten.  Generally you want to have a ¼” and 3/8” wrench (possibly a ½” wrench for bigger jobs) in your toolbox.  

Basically, you will need to remove the electrical connections from each of the spark plugs individually, then pull out the entire spark plug mechanism.  You’ll remove the spark plug by using a 5/8” socket.

The spark plugs fire in a specific regular sequence. Here is the schedule for a 3.3 liter V6 Dodge Caravan. Image courtesy of Rick Muscoplat.

You should make sure to do the plugs individually, one by one because they need to be plugged in a certain firing sequence.  If you mess up the sequence, your engine won’t start and you’ll have to refer to your owners manual to set it up again correctly. This is tedious.

Fouled Spark Plugs.  Image Credit: Brendan Falkowski via Flickr 

How to change spark plugs

Changing your own spark plugs will take you about an hour to complete. This guide is based on a four-cylinder engine. Spark plugs in different vehicles are different, but the process is basically the same.

1. Make sure your engine is cool

Make sure they engine is cool when you start.  Don’t do this after you’ve been driving somewhere. Give the engine some time to cool off. To see if your engine is cool enough, simply turn the ignition to start the power and check the engine temperature on the engine temperature gauge.

2. Locate the spark plugs

Your first task will be to locate the spark plugs. You should check the owners manual for your car for this.  When you open the hood of your car, you will see a bundle of wires, about 4-8 wires leading to different points on the engine.   If you have a four-cylinder engine, you’ll have four plugs, if you have a V8, you’ll have eight, four on the left and four on the right.

Follow thick, rubbery wires under the hood, and those will lead you to the plugs.

3. Remove the wires

Once you locate your spark plugs, you can begin removing the wires from the spark plugs.   Spark plugs fire in a particular order, so it’s important to remove the wires one at a time to avoid getting them mixed up.  Start at the end of one row and pull the wire off of the end of the spark plug, by clutching it as close to the end of the engine as possible, then pull it off my wiggling it a little.

4. Inspect the wires

Spark plug wires are durable.  Since they aren’t a moving part, they don’t wear out that often, and a quick inspection can help avoid any future problems. They are also easy to trouble shoot and you can easily rule them out as the source of engine problems.

The only thing that can really go wrong with the wires is a break in insulation, or the rubber on the outside of the wire which channels the electricity through the wire and delivers the spark into your engine.  You don’t want that spark anywhere else except for where it needs to be. This can cause a painful electric spark in other parts of the vehicle and also be a major fuel hazard, especially if you have a leak.  Unburned fuel can pass through the exhaust system, where it can harm your catalytic converter.  Your car will run rough, and of course this will affect your fuel economy. When you change your spark plugs, this is a good time to check your spark plug wires. 

What to look for

You are looking for any cracks in the housing or anything that is not smooth, bendable wire.  You should bend the wires to make sure the housing will not crack.   Start at the distributor end of the plug and check the whole way along through to the plug end.  Check the wires one at a time: pull the wire off the plug and inspect for cracks.  Make sure there are no burn marks or dark spots at the ends.

If you find any damages, just replace them or get them replaced.  Spark plug wires aren’t expensive, they can cost as little as $20 or over $100 depending on the vehicle. 

5. Loosen the plug

Now that you have one plug off, you can work on it.  Have your spark plug socket and extension on your ratchet ready to go. 

Use your ratchet wrench to loosen the plug.  Turn counter-clockwise to loosen as you fix the end of the plug, making sure to push it as far as it will go.  Pull out the old plug. 

6. Inspect the plug

Inspect the old plug. It should be slightly black, with a bit of soot on it.  If it’s white or oily, you may have other problems, so make sure to note how they look.  Make sure the porcelain or ceramic insulation is not cracked.  If you are unsure, save the plugs and ask your service advisor for advice. 

Make sure the new plugs are set up the same way the old plugs are.  Again, consider buying better quality spark plugs as they will last you much longer.
Note: Re-gapping the plug

Generally, re-gapping is something that was done on older cars and older plugs. These days, plugs already come gapped. Some say that you should still inspect and re-gap the plug no matter what, however, it’s not necessary but you can check the gap using the gap gauge tool. The ‘gap’ is usually 0.028 – 0.06 inches, and is specific to each vehicle.


Spark Plug gap gauge. Image Credit: Reluctant

7. Insert the new plug back in

When you can’t reach a nut or bolt with the wrench only, you use and extender, as in the case of spark plugs.  They give you more reach.  Put the plug wire end of the plug in the socket, and holding only the extension tool, guide the plug back into the hole.  Don’t bang on or force the plug, this can harm the gap or damage the plug.

Ratchet tool extenders: tools you can’t do without. Image from

Now screw the plug in. To keep from cross threading, thread the plug in by hand to start, until it stops, then insert the wrench on the end and tighten, making sure not to over tighten.  If you are using a torque wrench, you can control the torque precisely. If not, just make sure not to tighten too much as the metal is soft and can easily be damaged.

Now repeat the same process for all the plugs.  Once you’re done, start your engine, it should sound noticeably smoother than before. 

How do I know I’ve done it right?

You’ll know when you’ve done it right when the engine starts. If you messed up the plug wires by not doing them in order, your car simply won’t start or run really rough.  The worst that can happen is you’ll hear a loud backfire. 

When in doubt, don’t hesitate take it to our service professionals who will be happy to help you replace your spark plugs.

Further Reading:

Family Handyman

Auto Repair on Spark Plug Wire Removal


Rick Muscoplat 3.3 liter V6 Chrysler firing order

DownForce Motoring

Categories: Service, Parts

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