Spark plugs are instrumental in making a gasoline engine run. They shoot out a spark of electricity into the compressed mixture of air and gasoline within an engine's cylinders. This ignites the mixture and forces the cylinder's piston down. The motion of the piston is what creates power.
Spark plugs also have another job -- they pull heat away from the combustion chamber. That means spark plugs can get very hot. In general, cars have a spark plug for every cylinder in the engine. For instance, a four-cylinder engine will have four spark plugs. However, there are exceptions to the rule -- a vehicle with a HEMI engine will have two spark plugs per cylinder.
Spark plugs wear out over time. As they get older, they may not spark properly. This affects your engine's performance and results in a loss of power. But replacing your spark plugs isn't like other car repair projects -- it's much more straightforward and only requires a few tools. You don't need to be a skilled auto mechanic to change your car's spark plugs. In most cases, you can switch out an old set for new plugs in about an hour.
Before changing your spark plugs, you should consult your vehicle's owner's manual. You're looking for two things: how often you should change your spark plugs and where the spark plugs are located on your engine. For most vehicles, the rule of thumb is to replace your spark plugs every 30,000 miles (48,280 kilometers).
You should also make sure the engine is cold before you start -- spark plugs can get very hot! Even after other parts of your engine have cooled down, the spark plugs may still be too warm to touch. Let your engine cool down for a couple of hours before you begin.
Tools Needed to Change Spark Plugs in a Car
You'll need just a few tools to change out the spark plugs on your car. The most important tool is a socket wrench with an extension and a spark plug socket. Spark plug sockets come in two sizes: 5/8 inches and 13/16 inches. Most spark plug sockets have a rubber insert that holds the plug snugly in place.
You may also need a universal joint if your spark plugs are difficult to reach. Universal joints are articulated joints that allow you to turn the wrench in one axis and convert this motion into a rotation around another axis. If you can't maneuver a wrench and extension in the space around your spark plugs, a universal joint may help.
When replacing spark plugs, it's important to avoid over-tightening the plugs. Tightening a spark plug too far could damage the plug or the threads in the cylinder head. Use a torque wrench to make sure you don't over-tighten your plugs. Torque wrenches have a gauge that displays the torque you've applied when turning the wrench.
While most modern spark plugs come pre-gapped from the manufacturer, a gapping tool is good to have on hand. You use a gapping tool to make sure the gap between the spark plug's center electrode and the ground electrode above it are the right distance apart. This is where the spark fires on your spark plug.
You may also want to have on hand a pair of boot pliers, a length of rubber tube, a clean rag, some rubbing alcohol, a tube of anti-seize and compressed air or a wet/dry vacuum on hand. These items aren't critical for a basic spark plug replacement job but can help ensure that the process goes smoothly.
We'll walk you through the correct steps on the next page.
Removing Old Spark Plugs
Use your owner's manual to locate your engine's spark plugs at the top of each cylinder. Look for any dirt buildup around the base of the wire. Clean the area around the plug with compressed air or a wet/dry vacuum. This helps prevent anything from falling into the cylinder once you get started. You may also want to use a clean rag and rubbing alcohol to clean away some of the dirt.
Before you dive in, remember it's easiest to change your spark plugs one plug at a time. Accidentally plugging the wrong wire boot into the wrong spark plug can cause problems. It's easiest to avoid confusion if you remove and replace each plug individually. Once you're finished with the first plug, you can move on to the next one.
You should see a wire connected to the spark plug. Remove the wire by pulling on the boot -- the connector at the end of the wire. Spark plug wire boots are usually rubber or plastic. Do not pull on the wire itself or you may damage the connection between the wire and the spark plug.
If your spark plug wires have a rubber boot, grip the boot and twist it gently to the right and left. This should help loosen the connection. If a boot is particularly stubborn, you may need to use a pair of boot pliers to grip it and unplug the wire. Wires with a plastic boot tend to be easier to remove -- just grip the plastic and pull gently.
Once the wire is removed, you can use your socket wrench with the spark plug socket to remove the seated spark plug. Fit the socket around the spark plug and turn the wrench counter clockwise. If the cylinder is at an odd angle, use a universal joint to reach it. The spark plug should unscrew easily. If you encounter resistance, stop. Forcing the spark plug to turn could break the plug or damage the engine. It's better to visit a mechanic to remove a stubborn spark plug than to perform expensive cylinder head repairs.
In most cases, the spark plug should unscrew easily. Once you've removed the old spark plug, set it aside. Inspect the threads. If they appear dirty, use a clean rag and some rubbing alcohol to clean them. Now you're ready to insert the new spark plug.
Attaching New Spark Plugs
Before you insert the new spark plug, rub some anti-seize along the threads. Anti-seize will help prevent the spark plug from getting stuck in the cylinder. The next time you need to change your spark plugs they should come out easily. Don't get any anti-seize on the electrodes -- it can cause your spark plugs to fail.
If you can seat the spark plug by hand using the socket wrench extension and spark plug socket, do so. Gently turn the socket counterclockwise to seat it before turning it the other way to screw it into place. It should turn easily -- if you encounter resistance, stop and turn it the other way before trying again. Once the spark plug is snug, attach your extension to your socket wrench or use your torque wrench to tighten the spark plug so that the plug's washer is compressed against the mounting surface. Don't over tighten the spark plug.
Pick up the spark plug's wire boot and replace it. If you have a plastic boot, it should click into place when you plug it in. If your boot is made of rubber, you may want to rub some dielectric grease on the interior of the boot to make it easy to remove in the future. Once you're finished, you can move on to the next spark plug and repeat the procedure. That's all there is to it!
Replacing spark plugs remains one of the few engine maintenance items that the average person can tackle without a lot of knowledge and experience. Just follow our instructions and you should be back on the road with a new set of spark plugs in no time.
Learn more by following the links on the next page.
The 30,000-mile service is the first major check up on a vehicle. Is it worth the cost? Find out at HowStuffWorks.
Related HowStuffWorks Articles
- Top 10 Everyday Car Technologies that Came from Racing
- How Car Computers Work
- How Driverless Cars Will Work
- How Hypercars Work
- How Auto Transport Works
- How Automotive Production Lines Work
- Can you assemble your own car?
- What makes a digital car digital?
- What's new in synthetic oil technology?
- Will car repairs in the future financially cripple you?
- Automedia.com. "Spark Plug Basics." (Oct. 22, 2009)http://www.automedia.com/Spark_Plugs/ccr20041201sp/1
- CarBibles.com. "The Fuel and Engine Bible." (Oct. 21, 2009)http://www.carbibles.com/fuel_engine_bible.html
- Car Craft. "Choosing the Right Spark Plug for Your Custom Engine." (Oct. 22, 2009) http://www.carcraft.com/howto/116_0703_spark_plug_guide/index.html
- CarTalk.com. "Dear Tom and Ray." Feb. 10, 1996. (Oct. 23, 2009)http://www.cartalk.com/content/columns/Archive/1996/February/10.html
- Ganesan, V. "Internal Combustion Engines." McGraw-Hill Inc. 1994.
- How to Change Spark Plugs. "How to change your spark plugs in 10 easy steps." (Oct. 21, 2009) http://howtochangesparkplugs.weebly.com/index.html
- NGK Spark Plugs. "Tech Info - Spark Plugs Overview." (Oct. 22, 2009)http://www.ngksparkplugs.com/techinfo/spark_plugs/techtips.asp?nav=31000&country=US
- Sparkplugs.com. "Spark Plug 411." (Oct. 23, 2009)http://www.sparkplugs.com/faqmain.asp