Checking spark plug wires? Know the appropriate way

Changing spark plug wires is fairly easy and usually requires no tools. Each spark plug wire is connected to a spark plug, and brings the electrical spark to the spark plug in order to ignite the fuel inside the engine's cylinders. A damaged spark plug wire can make your car run rough and affect gas mileage.

Spark plug wires are pretty durable. They aren't a moving part so they don't wear out too often. A careful inspection of your plug wires can avoid any problems.

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The only thing that can really go wrong with a plug wire is a break in the insulation. The insulation (the rubber on the outside of the wire) keeps the electricity where it needs to be so it sparks on the inside of your engine, not someplace else before it gets there. If the insulation is cracked, the spark will jump off the wire, or arc, onto something metal under the hood.

An arcing plug wire can cause a weak spark or no spark at all in the cylinder with the bad wire. This makes your car run rough and can affect your gas mileage. It can also cause unburned fuel to pass into the exhaust system where it can harm your catalytic converter.

A good time to check your wires would be while you change your spark plugs (read more).

So do a quick inspection and save yourself some headache. Here's how:

• Open the hood and locate the spark plug wires. They can be black, blue, red or orange. Most are black. The number of spark plug wires most often corresponds to the number of cylinders your engine has.

• Don't take all the spark plug wires off at once, even if they're numbered. Remove the spark plug wires one at a time. They must go back on the distributor cap in exactly the same order they came off, or you'll have changed the "firing order" and your car may run badly or not at all. You can cause expensive damage by switching the order of the plug wires.

• With your engine off, start at the distributor end of the plug wire and work your way toward the plug end. You're looking for anything that is not smooth, pliable rubber.

• Bend the wires slightly to be sure no cracks appear.

• Check the boots at the distributor end of the wires to be sure they are not torn or cracked.

• Finally, check the wires at the spark plug end one at a time by pulling it off the plug and inspecting the end for any tears or cracks. Also look to be sure there is no burning or darkening of the end.

If you find any damage, it's time to buy a new set. They can be as little as $20 or as much as $100+ for a set depending on your application. It's worth the cost, though. A bad plug wire can be a little monster, it can even trigger your Check Engine Light.

Spark plug wires should be changed when they start to become hard or inflexible, or look dry and cracked. If one wire fails, it's probably time to change them all. Spark plug wire life varies greatly depending on a number of factors, but you should change them at least as often as manufacturer's recommendations. Cracked wires can cause misfires resulting in poor performance. This can also cause cross firing, which can result in pre-ignition. An accumulation of grease or oil on the wires may lead to early replacement, and lastly, corrosion at the spark plug terminal or distributor cap may also lead to premature failure.

Oh, and keep them clean, too! A thin film of dirt can form on the outside of the plug wire and around the boots at the ends of the wires can form conductive path to the high voltage when exposed to moisture. The conductive path can drain away the current that should go to the spark plug. If your car misses out or runs rough only in damp weather, you may have this type of wire problem.