Haynes Tech Tips - Spark Plug Replacement
Spark plugs light the fire in your engine’s cylinders, firing about 25 times per second during normal driving. If even one of these sparks is missed (called a misfire), you’ll feel it as a momentary stumble in the engine’s performance. While there are many causes of misfire, worn or dirty spark plugs are a common culprit. Additionally, worn spark plugs will cause your vehicle to run less efficiently, which means decreased fuel economy and power. It is generally a good idea to replace conventional spark plugs every 30,000 miles. Platinum or iridium spark plugs can last as long as 50,000 or even 100,000 miles. But again, more frequent replacement will ensure your engine is running its best.
Allow the engine to cool completely before attempting to remove the spark plugs.
- Purchase a service manual for your vehicle. It contains essential information for getting the job done safely and correctly the first time.
- If you don’t already have them, obtain the tools necessary for spark plug replacement.
Tools necessary to replace the spark plugs:
- Spark plug socket – This will have special padding inside to protect the spark plug’s porcelain insulator
- Torque wrench – It is important that the spark plugs are tightened to the recommended torque
- Ratchet – Standard hand tool to fit the spark plug socket
- Extension – Depending on model and accessories, you may need special extensions and universal joints to reach one or more of the plugs
- Spark plug gap gauge – This gauge for checking the gap comes in a variety of styles. Make sure the gap for your engine is included
- Determine if there are any special procedures required to access the spark plugs. Sometimes other components will need to be removed or repositioned. Your service manual will give the specifics for your vehicle.
- The spark plugs are located near the top of the engine. On some vehicles, you’ll have to remove an ignition coil for access to each spark plug. Other vehicles will have large wires attached to the spark plugs. Grasp the wire by the boot, rotate it and pull it off.
- If compressed air is available, use it to blow any dirt or foreign material away from the spark plug hole. This prevents debris from falling into the cylinder as the spark plug is removed.
- Unscrew each spark plug, using the special spark-plug socket, a ratchet and extension. Replace only one spark plug at a time.
- Check each of the plugs as you remove them - they will indicate the general running condition of the engine. In an engine running normally, the spark plug tip should be brown or grayish tan in color. If it is any other color, or if there are deposits on the tip, there is a problem with the way the engine is running. Your service manual should have a chart that will help you determine the running condition of the engine based on the appearance of the spark plug tip.
- The gap between the spark plug electrodes is a critical dimension that must be correct. Most spark plug gaps are set at the factory, but sometimes gaps can be wrong, even on new spark plugs. In most cases, you’ll want to check the gaps of the new plugs, assuring they are to the specification listed in your service manual. In some cases, particularly with iridium spark plugs, the manufacturer recommends against checking and adjusting the spark plug gaps. Always follow the recommendations of the spark plug manufacturer.
- Before installing the new plugs, apply a thin coat of anti-seize compound to the threads.
- Thread one of the new plugs into the spark plug hole until you can no longer turn it with your fingers, then use a torque wrench to tighten it to the specification listed in your service manual. It is a good idea to slip a short length of rubber hose over the end of the plug to use as a tool to thread it into place. The hose will grip the plug well enough to turn it, but will start to slip if the plug begins to cross-thread in the hole – this will prevent damaged threads.
- Attach the ignition coil or spark plug wire to the new spark plug, assuring it is fully seated on the plug.
- Repeat the procedure for the remaining spark plugs.
On vehicles with ignition coils over each plug, disconnect the electrical connector (A) and remove the bolt (B).
Unless the manufacturer recommends against it, you’ll want to check the gap on each spark plug.
Apply a thin coat of anti-seize compound to the spark plug threads, being careful not to get any near the lower threads.
A length of snug-fitting rubber hose will save time and prevent damaged threads when installing spark plugs.