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With today's vehicles boasting "100,000 miles between tune-ups", spark plug replacement as a routine maintenance practice is often overlooked or considered unnecessary. Truth is, regular spark plug replacement on older vehicles is still a necessity, and even newer vehicles will benefit from a periodic inspection. So if your skills in this area are a little rusty, now is as good a time as any to brush up on them.
The spark plug is responsible for generating the spark that ignites the air/fuel mixture in the cylinder during the compression cycle, so the expanding gases generated by the exploding fuel apply maximum force (power) against the piston. The spark plug uses the high-voltage electricity generated by the ignition coil to generate a spark of sufficient strength to ignite the air/fuel mixture. Voltage generated by the spark plug can range from 40,000 to 100,000 volts.
Normal wear, as well as fouling, burning, or other premature damage, can affect the spark plug's ability to generate a sufficient spark. This can result in engine performance problems (such as "misfire), poor fuel consumption, and excessive vehicle emissions, and may result in damage to other engine components or systems.
The style of plug, type of ignition system and your personal driving habits can all affect the spark plug replacement interval. While some newer vehicles equipped with platinum-tipped spark plugs can go as far as 100,000 miles between replacements, a good “rule of thumb” for most vehicles is to replace at 30,000 miles. Check the owner’s manual or service manual for your vehicle for the manufacturer’s recommendations.
Additionally, trouble in other vehicle systems (defective intake valve stem seals, excessively rich air/fuel mixture, or a defective oxygen sensor) can result in “fouling”, which can cause misfire or other “ignition-related” symptoms. In these cases, a “quick check” of the spark plugs is an easy an inexpensive starting point.
BE SURE you have the necessary tools and materials “on hand” before you begin. Depending on your specific situation, you’ll need some or all of the following:
BE SURE to note the routing of each plug wire before disconnecting, and BE SURE to re-route the wires accordingly when reconnecting. A misrouted plug wire can pick up interference from other vehicle electrical systems, and result in ignition problems.
NOTE: Remove, inspect and replace ONLY ONE spark plug at a time to avoid confusion when reconnecting plug wires. If you decide to disconnect ALL plug wires at the same time, use tape to label each wire with its proper location.
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A quick inspection of your spark plugs can tell you a lot about how your engine is running. Check your plugs for the following CONDITIONS. If any of the SYMPTOMS are noted, you should investigate further, using the POSSIBLE CAUSES listed as a guide.
Condition | Symptom | Possible Cause |
---|---|---|
Turbulence Burning | Worn insulator on one side of the plug | ● Normal turbulence in the combustion chamber
● Overheating (if plug also shows premature wear) |
Overheating | White or gray blistering of the insulator., excessive gap wear. | ● Improper spark plug (too "hot")
● Over-advanced ignition timing ● Detonation (spark knock) ● Cooling system malfunction ● Excessively lean air/fuel mixture ● Low-octane fuel ● Stuck (closed) heat-riser valve |
Glazing | Shiny yellow glaze over the insulator. When sufficiently heated, the glaze acts as a conductor, allowing current flow through the glaze that "shorts out" the plug. | Frequent acceleration to "wide open throttle" following prolonged periods of "low-speed" operation. |
Cold Fouling | Dry, fluffy, black carbon deposits on the tip of the plug. | ● Excessively rich air/fuel mixture
● An ignition fault that causes the spark plug not to fire ● Sticking valves (when only one or two plugs show evidence of cold fouling) To prolong plug life, BE SURE to correct the cause of the problem before reinstalling or replacing the plugs. |
Wet Fouling | Excessive oil contamination on the tip of the plug | ● Worn valve guide or valve guide seals
● Defective transmission vacuum modulator (automatic transmissions only) ● Worn rings (high-mileage engines) ● Excessive cylinder wear (high-mileage engines) To prolong plug life, BE SURE to correct the cause of the problem before replacing the plugs. |
Splash Fouling | Excessive carbon deposits on the tip of the plug | Generally occurs immediately following an overdue tune-up. Deposits accumulated in the combustion chamber (due to misfiring) loosen during high-speed driving, and stick to the hot insulator and electrode surfaces of the plug. In some cases, these deposits can "bridge" the gap between the electrodes and stop the plug from sparking. |
Pre-ignition Damage | Melted electrodes or chipped electrode tips | Excessive engine temperature due to:
● Over-advanced ignition timing ● Burned head gasket ● Low octane fuel ● Loose plug ● Using plugs of the improper heat range To prolong plug life, BE SURE to correct the cause of the problem before replacing the plugs. |
NOTE: Refer to the owner’s manual or service manual for your vehicle for the proper spark plug gap specifications. BE SURE to gap each plug properly, both NEW plugs and any plugs you plan to REUSE.
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