As long as you've got the wires off, you might as well do an inspection of the insulation. Check all along the length of the wire for tears or cracks. When you get to the end of the wire, where it attaches to the spark plug, check for dark or burned spots. Wires aren't a moving part and don't wear out often, but bad insulation could rob your engine of some useful spark.
Swapping Out Spark Plugs: An Overview
Changing spark plugs isn't too hard, even for the mechanically disinclined. If you're careful, you should have little trouble.
How do you know if your plugs need to be changed? The surest sign is on your odometer. Spark plugs usually need to be changed every 30,000 miles (48,280 kilometers). Some high-performance plugs can go as long as 100,000 miles (160,934 km) before replacement. If you don't know when yours were last changed or if you have an engine that runs roughly or has recently exhibited a decrease in fuel economy, well, that could mean that your engine might benefit from some fresh, clean sparks. As always, check the owner's manual to see what works best for your vehicle.
You'll need a spark plug socket for your socket wrench and a gap gauge. You can buy a spark plug socket wrench specifically made to fit your car's plugs or you can get a universal spark plug socket wrench made to fit the most common hex head sizes. As we've already said, you probably won't need to gap your plugs, but you may need a gap gauge to double-check that the space between the center electrode and the ground electrode is correct.
To find the plugs, simply locate the wires and follow them. There's usually only one plug per cylinder, but they fire in a specific order set by the manufacturer. Pick one plug to start with and gently remove only that wire. Changing one spark plug at a time is a lot easier than resetting the engine after you've replaced the wires in the wrong order.
Now whip out that new spark plug socket and put it on the end of your wrench. Plug sockets usually have a layer of foam inside to make this process easier. (It grips the spark plug.) If your socket doesn't have a gasket, use a little electrical tape inside the socket to get a better grip. Brush any debris away as you remove the plug. When the plug is unscrewed, just lift it out of the hole.
If you're going to gap, do so now. Your owner's manual should tell you where the gap should be set; set your gauge and slide it between the ground electrode and the center electrode. You want the electrodes to touch the gauge, but not too tightly.
Place the new spark plug in the empty hole using the plug socket. If possible, you may even want to remove the wrench and tighten the spark plug with your fingers. To make sure the threads are properly aligned, give the plug a couple of turns counterclockwise to seat it before tightening the plug by hand. Once the plug is finger-tight, you can finish the job with the socket wrench.
Connect the loose spark plug wire to the terminal at the top of the plug. You'll probably feel the wire snap on securely. When you've finished replacing the first spark plug and the wire is safely back in place, move on to the next plug in the row and repeat the entire process.
That was easy, right? Let's do some troubleshooting anyway.