How to Replace Spark Plug Wires

Changing spark plug wires are 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--here's how to replace them.

Difficulty: Moderately Easy

Instructions

Things You'll Need:

• Spark Plug Wires

1. 

   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.

2. 

   Open the box of new spark plug wires. If the new wires aren't labeled ("1," "2" and so on), lay them out in order of length.

3. 

   Start at one end of the engine and remove only the first spark plug wire: Pull on the boot (at the end) of the plug wire until it pops off.

4. 

   Follow the wire to its other end, where it's plugged into the distributor cap. Remove the wire from the distributor cap by pulling on the end of the wire.

5. Check the spark plug. A good spark plug will be lightly coated with greyish brown deposits. If heavy deposits are present, if the spark plug is black, or if the electrode or core nose are damaged, replace the spark plug (see "Replace Spark Plugs" under Related eHows).

6. 

   Note that most cars have small plastic pieces with slots that hold the wires in place so they don't rub against each other or rest on any part of the engine. Be sure to put the plug wires back into their slots to ensure longer life.

7. Replace with a new spark plug wire of the same length or number. The plug wire will make a quiet "pop" when it snaps on the spark plug.

8. Continue with the next spark plug wire, always taking one off at a time and replacing with a new wire until you've replaced all of the wires.

Tips & Warnings

• Remove and replace the spark plug wires one at a time.
• Although replacing the spark plug wires should be part of a 30,000-mile tune-up, many shops don't include this because the cost of spark plug wires brings up the cost of the tune-up.
• Some cars have spark plug wires that are permanently attached to the distributor cap. In this case, you must buy and replace both the cap and the wires at the same time.
• If the wires aren't old and only one needs replacing (maybe it has rubbed against something and is worn), you can purchase only one wire--you don't need to buy an entire set.
• Cheap spark plug wires aren't worth the money you save. Use a high-quality brand.
• Don't take all the spark plug wires off at once, even if they're numbered. Remove and replace 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.

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[tom12]

on 2/21/2009 Don't forget to put a little dielectric grease inside the end of the boot.

[tom12]

on 2/21/2009 If all the wires are bundled together and you can't remove them one at a time, use white-out or something else to mark numbers on them, and a digital camera to take pictures of both ends, so that you can keep track of which order they go in. (You can reach your arm behind the engine and take a close up photo even if you can't stick your head back there.)

on 2/12/2007 Plan on changing your wires every year or at least every 2 years, regardless of how often you drive the car. The wires' insulation will eventually dry out and crack, especially with the under-hood heat from the engine/exhaust, natural exposure to ozone, and so on. By the time you can actually see corona discharge from your plug wires, even in the dark, they're already bleeding off much-needed spark energy. In some cases, performance problems like bogging, poor acceleration, rough idling, etc., can be attributed to decrepit plug wires, and simply swapping out the wires can make you feel as if you just got a brand new car. All this is especially critical in cars with tight engine compartments where all that under-hood heat is bottled up with nowhere to go.

on 11/22/2005 If you sense there is a bad wire, while the engine is running take a spray bottle (an old window cleaning bottle) and pump a mist of water around the wires. If you have high resistance in the plug or wires, the spark will usually arc externally from the wire. It also helps if you have the engine in reverse while someone is inside holding the brake, making a load on the engine.

on 11/22/2005 To test spark plug wires, wait until it is dark outside, turn your car on and lightly rev your engine while another person watches your wires. If the wires light up or you can see sparks inside the wires while you do this, your wires are bad.