Up until the 1990s, vehicles needed a tuneup at about a 40,000-mile interval. Back then, this consisted of setting the idle speed; cleaning the choke and doing other tweaks to the carburetor; replacing the ignition breaker points (pre-1980); and replacing the spark plug wires, distributor cap, distributor rotor, coil wire, and spark plugs.
Today, of course, the breaker points, carburetor, and (in most cases) distributor and plug wires are all things of the past, and improvements in design mean that spark plugs only have to be changed about every 70,000 miles (or longer for iridium or platinum plugs).
Spark plugs have the job of igniting the fuel/air mixture in the cylinders, after the mixture is squeezed during the compression stroke. The plugs receive an electrical current from the coil and an electric arc jumps a small gap at the bottom of the plug, from electrode to ground electrode. A little explosion happens, forcing the piston down in the cylinder for the power stroke, and the combustion cycle begins again.
Why do spark plugs need to be gapped?
The distance between the plug's electrode tip and the ground electrode needs to be precisely measured, and it's determined by the amperage and voltage generated by the ignition coil. The thinking is that a smaller spark gap will develop a fatter, more intense spark, while a wider gap will produce a big, but weaker, spark. The gap is usually somewhere in the neighborhood of 0.025" to 0.035", but manufacturers' recommendations for specific engines can vary.
There's no need to be intimidated by the idea of setting your own spark plug gap. One tool used for this job consists of a series of wire loops that are very precise thicknesses; another is a metal disc with a beveled edge that is graduated from thinner to thicker.
Whichever tool you use, just insert it between the electrode and ground electrode of the spark plug and gently bend the ground electrode so that the two surfaces barely touch the feeler gauge as you pass it through the gap. You don't want the electrodes to drag on the feeler gauge, but you don't want them to clear the gauge easily either. The metal of the tip is pretty light, so it's important to be gentle. If the gap needs to be closed, gently press it against a bench or other solid surface. If it needs to be opened, pry very gently between the tip and electrode with a small standard screwdriver.
What happens when plugs aren't gapped properly?
With an ignition that is delivering less voltage, a narrow gap might deliver a weak spark, and with a "hotter" ignition, the narrow gap might deliver such a hot spark that it will wear prematurely. A wider gap runs the risk of putting out a spark that's so weak that it can't ignite the fuel properly, resulting in a miss or an intermittent miss, poor fuel economy, and poor power.
Remember that any spark plug will erode over time, as the electrode tip and ground electrode literally melt away from years of use. That's why it's a good idea to set the plugs' gap at the manufacturer's minimum recommended setting, as it will wear and widen over time. The introduction of rare earths like yttrium, iridium, tungsten, and palladium has resulted in spark plug electrodes that can last far longer than older designs.
When you remove the old plugs, they should have an even, whitish-beige color. Anything else, such as carbon or oily deposits, ash, or glazing, can indicate engine trouble.