Black-and-white close-up of three spark plugs

Pro Tips for Spark Plug Wire Replacement

Spark plug wire replacement has not gone out of style. As drivers keep their cars longer, replacement is needed eventually.

The spark plug wire conducts high-voltage pulses from the ignition coil or distributor to the spark plug, keeping that voltage, 10,000 to 30,000 volts, from arcing elsewhere. Spark plug wires wear out internally because this high voltage burns them out, and they deteriorate externally due to high heat, exposure and abrasion. Even the best mousetrap or cat can’t keep rodents from sneaking into your engine bay and chewing spark plug wires. The result could be rough idling, poor performance, poor fuel economy or illuminated check engine lights.

Due to age, mileage, wear or damage, spark plug wires need to be replaced to restore engine performance. But how can you tell if you need new spark plug wires, and what’s the best way to replace them?

Testing Spark Plug Wires

To test spark plug wires for failure or weakness, try the following methods:

  • Insulation Test: One quick and dirty spark plug wire test mimics driving on a wet day, which tests the insulation. With the engine running, spray the spark plug wires with water. If the engine stumbles, the insulation on at least one spark plug wire is leaking.
  • Resistance Test: To test the internal conductor, disconnect both ends of one spark plug wire, cylinder one’s, for example. Use a piece of string to approximate the length of the wire, then measure the string. Use a digital multimeter to measure the spark plug wire’s resistance. They typically measure 10 to 15 ohms per foot. If cylinder one’s spark plug wire is 1.5 feet long and measures 25 ohms or higher, the result is a weak spark and poor combustion. If its resistance reads “OL” or “∞” (infinite), it indicates an open circuit, which means there’s no spark going to cylinder one. Reconnect spark plug wire No. 1 and test the other wires.

Spark Plug Wire Replacement Pro Tips

Spark plug wiresReplacing spark plug wires is a matter of removing the old and installing the new, but don’t get ahead of yourself. Here are three tips to get the job done well:

  1. Spark plug wire order is critical. Each spark plug wire must be in order — distributor cap port one to spark plug one, and so on. Some spark plug cable sets come prenumbered, but not all are so convenient. The best way to replace spark plug wires is to change one cable at a time, routing each one exactly like the original.
  2. Spark plug pliers can prevent damage to spark plug wires. Use the pliers to grasp and twist the spark plug boot to free it, then pull away from the spark plug. Never pull the spark plug wire itself.
  3. Dielectric grease should be used at both ends of the spark plug wire, but don’t put a big gob in the end of the boot. Use a toothpick or cotton swab to apply a thin layer of dielectric grease to the inside of the spark plug boot, preventing water intrusion, corrosion and arcing (short-circuiting).

Some spark plug wires last upwards of 300,000 miles, but there’s no recommended spark plug wire replacement interval. It’s a good idea to check spark plug wires during spark plug replacement or at least every 30,000 miles.

Check out all the ignition system parts available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on spark plugs, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photo courtesy of Pixabay.

about author

Benjamin Jerew

Ben has been taking things apart since he was 5, and putting them back together again since he was 8. After dabbling in DIY repairs at home and on the farm, he found his calling in the CGCC Automobile Repair program. After he held his ASE CMAT for 10 years, Ben decided he needed a change. Now, he writes on automotive topics across the web and around the world, including new automotive technology, transportation legislation, emissions, fuel economy and auto repair.

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