How Spark Plugs Work

Spark plugs.
Spark plugs are one of the few things that an amateur mechanic can repair without much trouble. See pictures of car engines.
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As engines and their electronics become more complex, one of the few things left to hobbyists and auto enthusiasts who like a little grease under their fingernails is the ability to change their spark plugs. Although just about every other car repair out there takes a code reader and a college degree to diagnose and fix, spark plugs remain accessible and easy to understand.

The first reliable spark plug was invented in 1903 by Oliver Lodge. They're aptly named as well; spark plugs are simply insulated plugs that are screwed into an internal combustion engine's cylinder head to deliver the spark that ignites the mixture of air and fuel in the combustion chamber. Spark plugs also transfer heat away from the combustion chamber.


Basically, this is what happens: The spark plug sits at the top of the cylinder head. The piston first travels down the cylinder, drawing in a mixture of fuel and air. The piston then goes back up toward the spark plug, compressing the mixture. At the very last second, when the piston is at its fullest reach or top dead center (TDC), the spark plug sparks and ignites the mixture. The piston is forced back down to create power for the vehicle, then pushed back up again to clear out the exhaust. At that point, the process starts all over again.

A four-cylinder car will have four spark plugs; a six-cylinder car will have six and so on (though a HEMI engine has two plugs per cylinder). Now that we've got the basics down, let's talk about the kinds of spark plugs on the shelves at your local parts shop.


Spark Plug Basics

It seems pretty obvious that a spark plug provides the spark that burns the fuel, but its secondary role as a heat dissipater is equally important. A spark plug's ability to transfer heat to the car's cooling system is based on the length of the insulator nose and the materials used for the center electrode and the insulator.

Standard spark plugs in modern engines have a copper center electrode core surrounded by a nickel alloy, which you can see at the tip of the plug. Inside the plug, the center electrode is encased in porcelain, which helps transfer heat from the engine to the cooling system. Premium spark plugs make use of precious metals, like platinum or iridium, in place of the nickel alloy. These metals have higher melting points -- and higher prices to match.


Speaking of temperatures, spark plugs come in two basic varieties: cold and hot. Cold plugs work best in high-horsepower high-compression engines. They have less insulation, so more heat can be transferred away from the combustion chamber to the outside of the engine. This is no laughing matter: If the plug isn't cold enough for a particular application, it can't get enough heat out of the piston chamber. This can lead to pre-ignition, knocking, and permanent engine damage. If you aren't sure which spark plug heat range to use, err on the side of using a plug that's too cold rather than a plug that's too hot.

Hot plugs have more insulation and are found in most standard engines. The extra insulation keeps the plug's temperature high enough to burn off carbon deposits, which allows for more time between spark plug changes.

As gas prices climb higher, more manufacturers are claiming that swapping out old spark plugs for their premium plugs will boost any car's gas mileage. This is true -- but only to a point. The fact is, dirty, carbon-fouled, misfiring spark plugs will definitely lower a car's fuel economy and replacing them with shiny, new plugs will definitely improve fuel economy. Whether those plugs have exotic metals or nickel-alloy center electrodes doesn't matter quite as much as having the appropriate heat rating and gap between the center electrode and the ground electrode.

Speaking of those gaps, almost any plug you can find at the auto parts store will come pre-gapped for your engine. The days of setting the gap with a gauge are pretty much over, unless you're squeezing every last bit of performance out of your Saturday night hot rod. Engine modifications often mean you must gap new spark plugs, but stock engines with factory-approved, pre-gapped replacement plugs can usually go without adjustment.

Now that we know how a spark plug functions in the engine and the materials they're made of, let's find out what's inside these little guys.


Spark Plug Parts: The Top-to-Bottom Tour

At the top of the spark plug sits the connector, or terminal. This is where the spark plug wire attaches. The terminal connects inside the plug to the copper core of the center electrode, which is surrounded by insulation.

Next comes the hex head. This is where the socket wrench fits for tightening and loosening the plug in its hole in the engine. Just below this is a gasket that compresses tightly against the cylinder head. It's also known as a crush washer or the seat. Some plugs have a tapered seat, without an additonal seal. These are generally used in iron cylinder heads, while plugs with gaskets are usually found in aluminum cylinder heads.


The bottom half of the plug is threaded. This is the part that gets screwed, gently yet firmly, into place. A tiny bit of the center electrode juts out of the plug's lower end. And the whole thing is capped off with a ground electrode or ground strap. The spark that makes the engine run jumps the gap from the very end of the center electrode to the ground electrode. This is what ignites the air-fuel mixture that's been compressed by the piston.

The ground electrode is made of metal, with options ranging from stainless steel to titanium. It can come in several shapes as well, from notched or Y-shaped electrodes to triple electrodes with three little arms that seem to reach for the tip of the center electrode. As far as materials and shape of the ground electrode are concerned, you pretty much get what you pay for. High-end spark plugs made with exotic materials will cost more, but they'll also deliver better conductivity and spark.

Next, we'll learn what we all want to know: the basic rules for changing spark plugs.


Swapping Out Spark Plugs: An Overview

Changing spark plugs isn't too hard, even for the mechanically disinclined. If you're careful, you should have little trouble.

How do you know if your plugs need to be changed? The surest sign is on your odometer. Spark plugs usually need to be changed every 30,000 miles (48,280 kilometers). Some high-performance plugs can go as long as 100,000 miles (160,934 km) before replacement. If you don't know when yours were last changed or if you have an engine that runs roughly or has recently exhibited a decrease in fuel economy, well, that could mean that your engine might benefit from some fresh, clean sparks. As always, check the owner's manual to see what works best for your vehicle.


You'll need a spark plug socket for your socket wrench and a gap gauge. You can buy a spark plug socket wrench specifically made to fit your car's plugs or you can get a universal spark plug socket wrench made to fit the most common hex head sizes. As we've already said, you probably won't need to gap your plugs, but you may need a gap gauge to double-check that the space between the center electrode and the ground electrode is correct.

To find the plugs, simply locate the wires and follow them. There's usually only one plug per cylinder, but they fire in a specific order set by the manufacturer. Pick one plug to start with and gently remove only that wire. Changing one spark plug at a time is a lot easier than resetting the engine after you've replaced the wires in the wrong order.

Now whip out that new spark plug socket and put it on the end of your wrench. Plug sockets usually have a layer of foam inside to make this process easier. (It grips the spark plug.) If your socket doesn't have a gasket, use a little electrical tape inside the socket to get a better grip. Brush any debris away as you remove the plug. When the plug is unscrewed, just lift it out of the hole.

If you're going to gap, do so now. Your owner's manual should tell you where the gap should be set; set your gauge and slide it between the ground electrode and the center electrode. You want the electrodes to touch the gauge, but not too tightly.

Place the new spark plug in the empty hole using the plug socket. If possible, you may even want to remove the wrench and tighten the spark plug with your fingers. To make sure the threads are properly aligned, give the plug a couple of turns counterclockwise to seat it before tightening the plug by hand. Once the plug is finger-tight, you can finish the job with the socket wrench.

Connect the loose spark plug wire to the terminal at the top of the plug. You'll probably feel the wire snap on securely. When you've finished replacing the first spark plug and the wire is safely back in place, move on to the next plug in the row and repeat the entire process.

That was easy, right? Let's do some troubleshooting anyway.


Troubleshooting Spark Plugs

Removing old spark plugs can tell you a lot about your car's condition, especially around the electrodes at the tip, where the spark happens. This is what those old mechanics mean when they talk about "reading" the spark plug. Don't worry, though: It's a lot less mystical than gazing into a crystal ball.

Reading a spark plug can turn up a slew of issues, and we'll cover the most common ones here. For instance, if the spark plug only looks a little dirty, it's okay. That spark plug has been doing its job for at least 30,000 miles (48,280 km). If it looks glossy, the spark plug may be overheating. If it's white, the spark plug is too hot for the engine. In other words, it has too much insulation to burn away those deposits. That's easily remedied with a colder, more appropriate spark plug. If the old plug looks oily, you've got problems: Oil is somehow getting into the combustion chamber, where it doesn't belong. Spark plugs can be a good indicator of this, but new plugs aren't the solution.


Stop if you find that the spark plugs are sitting so tight in their holes that you can't wrench them out. Breaking a spark plug off in the cylinder head means you've got to drill it out or make another trip to the store for a special tool to remove the broken part. In the worst case, the engine's cylinder head would have to be pulled and taken to a shop for repair. Sometimes you can avoid these headaches by using a penetrating oil like WD-40. Let it sit for an hour or two and try unscrewing the plugs again. When you replace the plug with a new one, remember to use an anti-seize material like Thread Magic to keep from having the same problem in 30,000 miles (48,280 km).

Sometimes after replacing the spark plugs, you may find that the car backfires or runs really rough or won't start at all. This is usually because you didn't follow the handy tips above and you've put the spark plug wires back on in the wrong order. Check your owner's manual for the solution to the mistake.

For more information about spark plugs and other automotive parts, please see the links on the next page.


Spark Plug FAQ

What is the purpose of a spark plug?
Spark plugs are insulated plugs that screw into an internal combustion engine's cylinder head to deliver the spark that ignites the mixture of air and fuel in the combustion chamber.
How does a spark plug work?
The spark plug sits at the top of the cylinder head. The piston first travels down the cylinder, drawing in a mixture of fuel and air. The piston then goes back up toward the spark plug, compressing the mixture. At the very last second, when the piston is at its fullest reach or top dead center (TDC), the spark plug sparks and ignites the mixture. The piston is forced back down to create power for the vehicle, then pushed back up again to clear out the exhaust.
How often do you need to change spark plugs?
Spark plugs usually need to be changed every 30,000 miles.
How much does a spark plug cost?
Spark plugs alone (without labor) cost around just $10. Higher-end options can cost as much as $25.
How much does it cost to replace spark plugs?
You can expect to pay for the cost of the spark plug and about $100 to $200 in labor costs. The total typically averages between $200 and $300.

Lots More Information

Related HowStuffWorks Articles

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