Engine’s spark plugs create sparks that ignite the air-and-fuel mixture in the combustion chambers. Because the end of a spark plug is located directly in the combustion chamber, spark plugs are exposed to very high temperatures and pressures. In addition, the high voltage from the coil is sent through the spark plug to its end.
Electricity jumps from the center electrode across the air gap to the grounding electrode. The heat produced when the electricity jumps across the air gap can actually cause part of the electrode to burn or melt. Therefore, spark plugs wear out over time and need to be inspected and/or replaced periodically.
Worn or dirty spark plugs are the most common cause of sluggish performance and misfiring under acceleration. Spark plugs are normally replaced during any tune-up. In the past, spark plugs were cleaned rather than replaced if they had been used less than 12,000 miles.
Spark plugs in electronic ignition systems usually last up to 30,000 miles and are replaced when an engine is tuned up.
Removing Spark Plugs To inspect or replace spark plugs, you must first remove them from the engine. The first step to removing spark plugs is to disconnect the spark plug wires. Each spark plug has a spark plug wire and a spark plug connector attached to it. In most cases, the spark plug wire connector simply slips over the end of the spark plug. However, over time, it may get stuck to the spark plug. Therefore, the spark plug wire may not always pull off easily. To prevent damage to the spark plug wire, never pull on the spark plug wire itself. Instead, pull on the terminal boot that’s attached to the plug. If the boot is tight, twist it gently to loosen it from the plug.
If the boot is attached to the plug very tightly or if the spark plug wire is hard to reach, it may be necessary to use a special spark plug wire removing tool to prevent damage to the wire. This tool usually resembles a pair of pliers.
Keep in mind that when spark plug wires are reinstalled, they must be reconnected to the same plugs. This is necessary to keep the firing order in the proper sequence. For this reason, it’s a good idea to mark the spark plug wires so that you can reinstall them on the correct plugs in the proper sequence. Some technicians place a piece of masking tape on each plug wire, so they can write the cylinder number on the tape.
Once the spark plug wires are disconnected, remove any dirt and debris from around the spark plugs with compressed air. Once the area around the spark plugs is clean, you’re ready to remove the plugs from the cylinder head.
Remember that the spark plug insulator is usually made of porcelain.
The porcelain insulator prevents electricity from leaking out of the plug. However, porcelain is very fragile. Therefore, when removing or installing a spark plug, be careful not to damage or break the insulator. To prevent the insulator from being damaged, a specially designed socket wrench called a spark plug socket is used to remove a spark plug The spark plug socket closely resembles a traditional deep socket.
However, unlike a deep socket, the spark plug socket has a soft rubber bushing inside that fits around the insulator to protect it from damage.
As you learned, the spark plugs used in modern automobiles vary slightly in thread size, length, and design. However, the hexagonshaped area of the spark plug comes in only two common sizes—5 8- inch and 13 16-inch. Therefore, spark plug sockets are available in both the 5 8- and the 13 16-inch size.
As you remove the spark plugs, keep the plugs in the order in which they were removed from the engine. For troubleshooting purposes, it’s important to know which plug came from which cylinder. If you discover a problem with one of the plugs, you may need to know which cylinder the bad plug came from in order to correct the problem.
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