How to Replace Spark Plugs

Save big on car maintenance costs by changing spark plugs yourself!

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Change your spark plugs yourself to maintain peak performance and high gas mileage. In most cases it's a simple job as long as you have the right tools.

Tools Required

  1. gap gauge
  2. Needle-nose pliers
  3. Rags
  4. Socket/ratchet set
  5. spark plug wire puller
  6. swivel socket.
  7. Torque wrench

Materials Required

  1. Anti-seize compound
  2. Spark plugs

What Does a Spark Plug Do?

First you need to understand what do spark plugs do? In short, they create a spark of electricity that ignites fuel and drives a car’s pistons, ultimately starting the car.

You already know that spark plugs wear out. Well, “burn up” is more like it, because when a spark jumps the gap between two electrodes, it actually burns off (erodes) minute amounts of metal from each one. Over time, the gap grows to the point where the spark can no longer make the jump. That’s when you get misfires, poor gas mileage, lousy acceleration and, ultimately, the dreaded “Check Engine” light. Read on to learn how to change spark plugs. We’ll also explain when to change spark plugs and how often to change spark plugs.

To keep vehicles running at peak performance for longer service intervals, many car manufacturers install extended-life spark plugs. Because their electrodes are coated with precious metals that have higher melting points, these plugs can sometimes maintain a precise gap for up to 100,000 miles. But even with higher melting points, metals like yttrium (2,779 degrees F), platinum (3,222 degrees F) and iridium (4,429 degrees F) can’t stave off erosion forever. The electrodes eventually erode, increasing the gap, and, well, you’ve already heard the rest of this story.

If you’ve changed your own spark plugs in the past but are intimidated by the newer-style coil-on-plug (COP) ignition systems (pretty much the standard since 2000), it’s time to reconsider. COP systems may look complicated, but they’re actually easier to work on than the older, distributor-based systems. Sure, you’ll have to learn a few new tricks, but the basics are still the same.

How to Replace a Spark Plug

Changing spark plugs takes about an hour (for a four-cylinder engine) and will save you at least a hundred bucks in labor. You can use the same old tune-up tools (ratchet, spark plug socket and gap gauge). You should use a torque wrench to tighten the plugs. But there’s a way to get around that if you don’t have one. Just follow these steps and you’ll be tuned up in no time.

When to Check & Change a Spark Plug

So how often should spark plugs be replaced? A common question for many vehicle owners is “When should I change my spark plugs?” Well, not all spark plugs are rated for 100,000 miles. In fact, some carmakers recommend replacement at 30,000-mile intervals. So always follow the spark plug service intervals shown in your owner’s manual to figure out when to replace spark plugs. But if you can’t remember when you last changed your spark plugs, you can pull them and check the gap and their condition. Once you’ve put in the labor to begin checking spark plugs, however, you may as well change them and establish a new baseline for the future.

Check out our favorite car diagnostic tools for DIYers here.

Signs of a Bad Spark Plug

Unlike manufacturers’ guidelines for oil changes, which are overly cautious, the recommendations for how often to change spark plugs tend to be overly optimistic. For example, if you’ve already got 80,000 miles on a set of 100,000-mile plugs, they’re 80 percent worn and beginning to take a toll on engine performance and gas mileage. Worse yet, after that many miles, there is a great amount of spark plug wear and spark plugs have a tendency to seize in the cylinder head. Removing a seized plug can be a costly job, especially if the threads in the cylinder head are damaged in the process. So what are the benefits of changing spark plugs? When you consider the gas mileage falloff and the possibility of seized plugs, early replacement makes sense.

Do it yourself or take it to a pro?

The answer depends on the type of engine in your vehicle. Some of the V-6 models have very difficult spark-plug replacement procedures that require removing portions of the intake manifold. If you’re not comfortable with that level of disassembly, you should take your vehicle to a pro. But if you have an engine with easy access to the rear bank, then you can probably do the job yourself. Just be sure you gap the spark plugs properly and use a torque wrench.

The tools shown are available at online suppliers and auto parts stores. While you’re there, ask the clerk for the spark plug gap and torque specifications for your vehicle. And buy a small packet of dielectric grease. It’s also important to ask about spark plug replacement cost before making an appointment.

Checking spark plugs: Spark plug misunderstanding

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When you change the spark plugs, don’t be shocked to see the center electrode “worn down” to the size of a pin. If your car was equipped with fine-wire iridium spark plugs, the center electrode on those plugs can be as small as 0.4 mm when new. You’re probably used to the much larger 2.5-mm diameter on a traditional copper spark plug center electrode.

We talked with Dave Buckshaw, Technical Trainer for Autolite Spark Plugs, to get the skinny on the latest spark plug technologies. Dave’s advice was simple: Regardless of which brand you buy, spend the extra money for fine-wire iridium-tipped spark plugs. These plugs cost a little more than platinum plugs, but they last longer and provide a much better spark.

For longer spark plug life, buy an iridium plug with a platinum-enhanced side electrode. They’re available at all auto parts stores.

Project step-by-step (6)

Step 1

Remove extras and clean your work area: Blow away the crud

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Start by removing the plastic “vanity” cover (if equipped) and the air cleaner assembly from the top of the engine. Be sure to label any vacuum hoses you remove so you get them back in the right place. Then clean the top of a four-cylinder engine, or the banks on a “V” engine, before you remove other parts

Blast compressed air around the ignition coils to prevent crud from falling into the cylinders. Then blow any remaining loose dirt off the engine before you set out your tools and new plugs.

Step 2

Remove the ignition coil and/or boot

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Disconnect the ignition coil electrical connector by depressing (or pulling up) on the locking tab. Then rock the connector off the coil. Next, remove the coil hold-down bolt and pull out the entire coil and boot assembly. Some COP systems have a detachable rubber boot and spring. If they don’t come out with the coil, retrieve them with needle-nose pliers and replace them with new parts. Then remove the old spark plug.

If your car doesn’t use COP ignition, simply pull the boots off the plugs.

Spark plug tool: Spark plug wire puller

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If your car doesn't have COP ignition, the spark plug wire will end in a boot that attaches to the spark plug. A spark plug wire puller makes it easy to pull the boot off.

Buy this handy tool on Amazon here.

Pulling the boot with a spark plug wire puller

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Wait for the engine to cool before removing plugs. Using a spark plug wire puller, grasp the boot as far down on the plug as possible, twist and pull straight out.

Step 3

Unscrew the plug

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Blow away the dirt and crud that’s settled on and around the plug since it was installed. Slide the proper size spark plug socket over the plug. A swivel head spark plug socket makes the job much easier. You’ll probably need an extension of some length to reach the plug. Rotate the plug counter-clockwise to loosen it.

Not all engines leave the plugs as accessible as shown here. The more compact the engine compartment, the harder it will be to get to the plugs. But all plugs can be removed.

Swivel socket and bent handle ratchet

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The right tools make removing the spark plugs simple.

Step 4

How to Gap a Spark Plug

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Gap all plugs before installation using the manufacturer's specs. Always check the spark plug gap before installing it. Autolite expert Dave Buckshaw recommends using a wire-style gap gauge instead of the inexpensive variable-thickness “disc-style” gauge. They can bend the electrode off-center or even break it off.

Slide the correct wire gauge (or gap gauge) between the electrodes. The wire should drag slightly between them. If the gap is too small, open it with the gap gauge. If the gap is too large, tap the side electrode lightly on a solid surface. Place a small dab of anti-seize compound on the plug threads and hand thread the plug into the cylinder head.

Opening and closing the gap

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Hook the adjusting tool onto the side electrode and pry it up slightly. Then recheck the gap. To close the gap, simply push the electrode down instead of pulling it up. Always recheck the gap after making adjustments.

Gap gauge

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Get the gap just right with an inexpensive gap gauge. It is less than $10 on Amazon.

Step 5

Install the new plug: Use the proper amount of torque

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Proper spark plug torque is CRITICAL in today's engines. Always use a torque wrench and the manufacturer's spark plug torque specs! Insufficient torque can result in a plug blowing right out of the cylinder head, taking the threads with it. Too much torque distorts the plug. If you used anti-seize compound on the plug threads, reduce torque by 10 percent. If you don’t have a torque wrench, go to the spark plug manufacturer’s website to find manual tightening techniques and spark plug torque specs.

Step 6

Lube the spark plug boot and button it up

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Apply a thin coating of dielectric grease around the inside of the spark plug boot before reinstalling the coil. The grease prevents misfires and makes it easier to remove the boot in the future. Then reinstall the ignition coil, hold-down bolt and coil electrical connector. Finally, reinstall the air cleaner and vanity cover and fire it up. Enjoy the extra power and gas savings.

Follow the same procedure for non-COP ignition system boots. Next, check out 100 car repairs you don't need to go to the shop for.

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