Servicing Your Spark Plugs

Automotive Maintenance Tips

A spark plug provides a means of delivering a timed electric charge to a cylinder just before it finishes compressing the fuel/air mixture. The electric current jumps a gap between the center post electrode and the grounded side electrode of a spark plug. This action ignites the combustive gas as the piston reaches Top Dead Center (TDC) and the resulting explosion drives the piston down which turns the crankshaft.

  • Spark plugs can become fouled or worn-out. Damaged spark plugs indicate serious engine problems. Fouled or worn plugs can cause vibration, introduce drivability issues and create poor gas mileage. Check your spark plugs for issues at least once per year. Spark plugs can offer tell-tale clues to engine performance issues or problems.
  • Replace your spark plugs regularly. The replacement schedule will depend on your car or truck manufacturer's specifications. It is usual that spark plugs should be replaced within every 30,000 miles, but varies as much as from 10,000 miles to 100,000 miles for scheduled spark plug replacement. This maintenance maximizes power, performance, and fuel economy.

    • Required Tools for Service:
  • Spark Plug Socket
  • Any Socket Extension(s) Required
  • Ratchet Wrench
  • Torque Wrench
  • Spark Plug Gap Tool or "Taper Gauge"
    (As recomended by manufacturer)

    • Required Specifications for Make/Model/Engine:
  • Spark Plug Gap Size
  • Spark Plug Requires Anti-Seize or Not? If so, How Much? Where?
  • Spark Plug Torque Requirements for your Make & Model of Engine.

    • Spark Plug Removal:

    You may have to prep your work area (the engine to work on) to allow you access to the spark plugs (such as possibly removing an air cleaner for better access). Some instances may require working in tight spaces or require a particular type of socket extension.

    When removing spark plug wires, pull on the spark plug cap, or "boot", and not the spark plug wire itself, to expose it. It is a good idea to mark each spark plug wire with masking tape and a marker according to which cylinder spark plug it was removed from for easy reference when reinstalling them, later.

    Use the appropriate spark plug socket wrench, or a spark plug socket and a ratchet wrench to loosen and remove each plug. In a pinch you can use a deep well (5/8" or sometimes 13/16") socket and ratchet wrench, but a spark plug socket uses a rubber insert to hold the plug and protect the porcelain insulator from cracking, but a spark plug socket or socket wrench is always preferred and highly recomended.

    As you loosen each spark plug, use a good shot of compressed air around the base of the plug to blow away any excess dirt or debris you may have jared loose. This will ensure that nothing should fall into the cylinder as you remove, or later reinstall, the spark plug. As you remove each plug, also mark it with a Sharpie to keep track of each spark plug's matching cylinder. This can help you identify any problem cylinders. Be sure to remove any sealing washer that may exist, as well.

    It is very important to inspect your spark plugs at this point.

    Spark Plug Inspection:

    It is very important to "read" your spark plugs. They can tell you much about how your engine is performing.

    A mechanic can diagose many problems by looking at the engine's spark plugs. All spark plugs removed from the same motor should look the same. If they aren't alike, a problem exists that should be addressed in the cylinder that the different spark plug was removed from.

    Be sure to pay attention to the condition of the spark plugs as you remove and mark them. The state of a spark plug is very important, as mentioned previously. Be sure to note any particular conditions that exist. Spark plugs will usually show normal burning, which is good. But the condition of a spark plug can also indicate cold fouling, gap bridging, glazing, overheating, preignition damage, splash fouling, turbulence burning, wet fouling, or even prove engine damage. You may have to consult with a mechanic if you see anything other than a normal firing plug.

    Worn and/or dirty spark plugs may idle fine and work well at low speeds, but they frequently fail during heavy loads, such as towing, or at highway speeds when the engine is working much harder.

    A spark plug indicating a normal burn process is colored light tan or gray, has little erosion on the electrodes, and should have very few, if any, deposits on it.

    Use a spark plug gap tool, or a "taper gauge" (according to your manufacturer's recomendations) to measure the gap between the spark plug center electrode and all grounding electrodes (modern spark plugs are often equipped with more than one grounding electrode). The gap should be the same on all grounding electrodes and on each spark plug. If variations between the gap exist on the same plug of spark plugs with more than one grounding electrode, spark will occur only across the smallest gap. Variations in the gap between different spark plugs could explain differences between plugs that look different from different cylinders, as all spark plugs should be set with the same gap. Obviously, setting the gap of a spark plug correctly is critical to your engine's performance. Luckily, it is easy to do. You simply use the spark plug gap tool's electrode gripper to pry open or closed the grounding electrode, as required, to establish the required gap clearance.

    When checking your current set of plugs, be sure to check that the electrodes are not burned, and the increase of the gap between electrodes should not exceed 0.001 inch (0.0254 mm) for every 10,000 miles (16,000 km) since the last time the spark plugs were serviced. Any spark plug exceeding these wear limits should be replaced and the cause of excessive wear diagosed, found and corrected.

    Additionally, if you are going to replace any one spark plug of the current set, we recomend that you replace the entire set.

    Spark Plug Replacement or Reinstallation:

    If you are replacing the current set of spark plugs, be sure to check the gap on the new plugs, as well. Sometimes the same plug will be used in a variety of engine applications and one motor could require a different spark plug gap size than another.

    The appropriate service manual, or your local auto parts sales store, can tell you what the gap for your spark plugs should be set at for your vehicle.

    Although spark plugs with multiple grounding electrodes should still be checked for accuracy, manufacturers require that the gap be adjusted with special tools and therefore ask consumers to leave the gap alone. If you find one that is out of tolerances, return it to the auto shop where you bought it for an exchange.

    When re-using gasketed spark plugs, replace the old gasket with a new one.

    Once the spark plugs are properly gapped, before they are replaced or new plugs freshly installed, check your manufacturer's spark plug servicing guidelines. On many late model vehicles, manufacturers recommend that an anti-seize compound be applied to the top two-thirds of the spark plug threads. You should check on your manufacturer's requirements, as the anti-seize compound must be applied in the correct amount and at the correct place. This is particularly important when required by the maufacturer, as it is engineered into the service considerations for the plug. Too little compound will cause an excessive gap in the contact between the spark plug threads and the spark plug bore. Too much may allow the spark to jump to the buildup of anti-seize, rather than jumping to the spark plug electrode.

    Before (re)installing the spark plugs, wipe away excess oil, dirt or debris from around the spark plug hole and from the seats of re-used plugs. If using the same plugs, you will want to replace them in the cylinder they came from. This allows you a true measure of the condition of that particular cylinder. This way, if you are required to read it again, all information will be from the proper cylinder, and not a mixed mash tale of how two different cylinders are doing at two different times.

    Make sure the threads of the spark plugs match the threads of the plug bores in the cylinder head(s). Take extra care not to cross-thread them. Always start the spark plugs into their cylinder head threads with your fingers to avoid cross-threading. Cross-threading a spark plug into a cylinder head may well likely destroy the functionality of the cylinder head. Save yourself some money, be logical and safe... Don't force anything.

    It is important to use a torque wrench to torque the spark plugs into the cylinder heads at the proper requirement as per your manufacturer. When spark plugs are not properly torqued into the cylinder heads, the threads of the spark plug may not make good contact to the head, or could damage the spark plug. Either way could upset the circuit or possibly even create an unwanted hot spot within the cylinder head. Always use a torque wrench to tighten spark plugs to their specified torque. Many of today's engines have aluminum cylinder heads. Not only do these heads often require different torque specifications than iron heads, they also require extra care when installing the plugs.

    We strongly recomend that you look up the torque specifications of your make and model's engine for the proper spark plug torque setting. Torque is measured in foot pounds of force. Be sure to get the right number. Your local auto parts shop can look this up for you, easily.

    Hooking-up the Wires

    Often, the stock spark plug wires are replaced when replacing the spark plugs. If you are replacing the spark plug wires, I recomend replacing them with high performance spark plug wires, as they not only perform better, they last longer as well, giving much more bang for the buck.

    When replacing wires, you will often purchase a "set" of spark plug wires for your car. Be sure to get the right set for your engine application and take some extra time to assure that you get the right wire on the right cylinder AND distributor connections. Use your old wires as a guide.

    In any event, be sure to get all the wires hooked-up to the right spark plug by pressing the boot firmly down on the spark plug.

    Now, pat yourself on the back, you're done. ;)


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