Change Spark Plugs

Spark plugs make a gasoline engine run by shooting sparks into and igniting the mixture of air and gas in the engine's cylinders, causing the cylinders' pistons to move down and create the power that runs the car. After a while, spark plugs wear down and need to be replaced. Unlike most car repairs, replacing your own spark plugs does not require any special tools or expertise. Read on to learn how to replace your plugs and have your engine running more smoothly in no time.


Locating the Plugs and Getting the Equipment Ready

  1. Figure out your spark plugs' location and size. Almost all cars have spark plugs located in a row at the front of the engine (or possibly on the top depending on configuration). Some older V engines have spark plugs located to the sides of the engines. With a few exceptions, cars have one spark plug for each cylinder.[1]
    • Make sure the engine is cool before you get started.
    • If you're having trouble locating the spark plugs, look for the black rubbery wires running around the engine. Follow them to the end and you'll find the spark plugs. Otherwise, some modern engines have a cover over the coils and plugs. The cover must be removed for access. This set up typically has the plugs going right down the center of the valve cover.
    • Check your car's owner manual for the exact location of the spark plugs and the spark plug size you need.
  2. Get your tools and supplies ready. Once you've determine the size of your plugs, gather the tools and supplies you need. You just need a few things to change your spark plugs. Get a socket wrench with a spark plug socket extension that's the right size for your plugs, as well as a new set of spark plugs.
    • In the past, a gapping tool was always required to fit the plugs properly, but modern spark plugs now come pre-gapped.[1] However, spark plug gaps may still be set incorrectly (even if preset). You should always check and set the gaps to be sure.
    • Having compressed air, a rag, and rubbing alcohol on hand can help the job go more smoothly.
    • If you want to make sure the plugs unscrew easily next time you need to replace them have a bottle of anti-seize on hand to apply a light coat over the threads of the plug before screwing it on.
  3. Clean the area around the spark plugs.[2] Any dirt that has built up around the plug will fall into your cylinder when you pull out the plug. Nothing should go into that hole while the plug is out, so make sure the area is clean. Use compressed air to blow away dirt and debris around the spark plugs, to make it easier to see what you're doing. If there's a lot of buildup, clean it off with a rag and some rubbing alcohol.

Removing the Old Plugs

  1. Pull out the first spark plug by the boot. The boot is the connector that connects the spark plug wire to the engine. Grasp it and carefully pull it out to detach the spark plug.
    • Do not pull out more than one spark plug at once. Each spark plug must be connected in a specific place. If you pull them all out at once, you might have trouble figuring out where to reattach them, and that could lead to big trouble for your car. If you want to pull out more than one at once, write down or label each spark plug wire before you take it off. This is important, as on an 8 cylinder engine, you can easily forget which wire goes where.
    • Do not pull the spark plug out by the wire. Make sure you grasp and the boot. If you pull on the wire you could damage it, and new wires cost a few hundred bucks to replace.[1] Twist the boot to aid in removing the spark plug boot from the plug.
    • If the boot is hard to pull out, you may need to use a pair of boot pliers to nudge it out.
  2. Inspect the well in the valve cover. Spark plugs that are installed in center of the head, often through a well in the valve cover, need an extra step for inspection. When removing the plug boot, inspect the end to see if it is covered in oil. If not, look down the well with a flashlight and check for oil as well. If oil is found on the spark plug or valve cover, a valve cover reseal might be in order. You don't want oil to get in the cylinder, for damage may occur.
  3. Unscrew the old spark plug. Fit the socket wrench with the spark plug socket, and place the socket around the old spark plug. Turn it counterclockwise to unscrew the socket. It should come off easily. If you feel resistance, stop, move the wrench backward, and start over; forcing the plug to unscrew could cause damage to the setting.
    • When initially trying to loosen a plug, it could take some force to break it loose. If the vehicle is equipped with iron heads, and the plugs have been installed for a while, there could be some resistance along with some noise while removing the plug.
    • If the engine is equipped with aluminum heads, the plug should break lose and then come out easily. If the engine has aluminum heads and you are using a lot of force to remove the plug, stop and screw it back in, as it means the threads are damaged already. There is a good chance that the head might have to be removed to be repaired.
  4. Clean the threads. The area where the spark plug screws into the wires should be free of dirt and debris before you install the new plug. Use rubbing alcohol and a rag to wipe away any grime before you continue.
    • Take a look at the electrode end of the old plug. Check to see if there is a carbon buildup on the electrode, or if the gap is filled with dirt. If it is oily, you may have worn rings, and you probably need to take your car to a mechanic to get them replaced. If the plug is tan, the engine is working properly.
    • Take a look at the porcelain in the center of the plug and look at the color. On most modern cars the color should be tan or an off white.

Installing the New Plugs

  1. Double check that your replacement plug is the same size as the old plug. The threaded part should be the same length, and the threads should match. If you're unsure what plug to buy, take an old one with you to the store and have the counter person match it for you. Also, they may be able to tell you the correct plug gap for your application.
    • Even though new plugs come pre-gapped, you might want to double check the gap of the new plug. It should be within the engine manufacturer's specifications. Plug gages are available at your local parts counter, usually for a dollar.
  2. Seat the new plug. Apply a little anti-seize on the threads so that the plug will unscrew easily next time you want to change it. Don't get any on the electrode, since that may cause it to fail. Screw the plug on by hand, twisting it clockwise, until it stays in place. Use a torque wrench fitted over the spark plug socket to finish screwing it on until it's fairly tight.
    • Do not over screw the plug; if you tighten it too much, you could cause damage to the part or the head.
    • There are some engines for which a torque wrench is absolutely necessary, because the spark plugs are indexed. Not getting the electrode into the proper position can cause damage. These engines often require plugs be purchased from the dealer so proper spark plug indexing is achieved.
    • The plug's washer should be compressed against the mounting surface.
    • For spark plugs in hard to reach areas, a piece of vacuum hose attached to the top of the plug will help align the plug and avoid cross threading as the plug is screwed in.
  3. Replace the plug wire. Add a small amount of dielectric grease to the inside of the boot. Pick it up by the boot and place it back where it came from. The boot should snap in snugly. Give it a tug to make sure it stays in place.
  4. Change the remaining plugs. Working one plug at a time, use the same method to finish replacing all of the plugs.
  5. Consider replacing your spark plug wires. Inspect the wires for gouges or rips. If they look like they're in bad shape, look into changing them, too.


  • Always replace all spark plugs at the same time.
  • Before replacing your spark plugs, it is recommended to clean the engine at a carwash or in your driveway with a spray bottle of Simple Green, a brush, and garden hose, paying special attention to the spark plug location areas of the engine. This will prevent dirt and grit from getting into the plug holes while replacing the plugs.
  • Always replace all spark plug wires at the same time.
  • A good spark plug socket has a rubber insert to hold the spark plug from falling out. There are two sizes: 5/8" and 13/16"


  • Putting spark plug wires back on the wrong plugs will cause a poorly running engine, and may make you think you have bigger problems than you do. If you just removed plug wires, and now the car runs very roughly, you should re-check the location of the plug wires against a service manual to be sure they're in the right place.
  • Make sure you gap your plugs to the specified gap, unless the plugs say do not gap. Do not try to change the gap of a plug that says do not gap.
  • Get the right size plug. If the thread depth of the plug is too long, it may come in contact with your piston, which will cause catastrophic engine failure. Don't guess which size, ask your parts guy... He knows, or at least has a computer or book that does.
  • Leaving a spark plug loose in your engine can cause it to shoot out of the engine like a bullet. Consider the tremendous pressure that your engine produces in each cylinder. If the plug is loose, it will strip out go flying at a high rate of speed.
  • If your engine has aluminum cylinder heads, the plugs must be replaced when the engine is cold. Installing plugs when aluminum heads are hot can cause the threaded area to crack. Be sure to apply anti-seize to the spark plug threads.

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