Spark plugs are like a lot of other items on a car -- they tend to get ignored until they quit working. This is bad. Long before spark plugs stop working entirely, they usually begin to give poorer mileage. And with today's gasoline prices, this fact alone should provide a car-owner with enough motivation to replace those old spark plugs with new ones at the interval recommended in the owner manual or shop manual.

That interval varies from car to car, but by checking the lubrication and maintenance schedule for your car, you can find out the exact mileage interval. One maker, for example, recommends a new set of plugs every 18,000 miles for several of its cars.

It doesn't hurt to change plugs either -- but never should you let plugs stay in past the recommended interval.

Another good idea is to remove the plugs about halfway through their recommended change interval and check the electrode gap. For example, if your owner or shop manual recommends a fresh set of plugs every 18,000 miles, at 9,000 miles you would remove the plugs and check the gap. The gap between the electrodes tends to widen as the plugs accumulate mileage.

Regardless of whether you're removing spark plugs to check them or to replace them with new ones, the removal procedure is the same: First Let the engine cool off, so you won't burn your hands against any hot metal parts.

If you have a four-cylinder or six-cylinder car, the spark-plug wires may be easily accessible. In this case you can remove them with your hand from the spark plugs. On many cars, though, especially V8s -- but on many smaller engines too -- it's hard to reach the spark-plug boot (that's the end of the spark-plug wire that fits over the spark plug).

In this case you need a special tool to remove spark-plug wires. It's inexpensive, and available at auto-parts stores and other places that sell tools.

Regardless of whether you use your hands to remove the spark-plug wires from the plugs or the wire-removal tool, the procedure is the same. Rotate the boot back and forth until it brakes free from the plug. Then lift the boot off the plug.

An easy way to keep from getting the spark-plug wires crossed is to remove only one plug wire at a time. Remove the spark plug, replace it and attach the wire before removing the next plug.

To reattach the spark-plug wire, simply push the boot back down over the plug. Never push or pull on the spark-plug wire (also called a cable) itself. pPush or pull only on the spark-plug wire boot.

To check the electrode gap of a spark plug you need a special gauge for checking the distance between the electrodes. You should use a round-wire spark-plug gauge. It will have several different thickenesses of wire attached to it, because different car-makers recommend different electrode gaps. You can buy a spark-plug gauge at auto-parts stores and many other places that sell auto parts and tools. It's inexpenseive.

Consult the shop manual or owner manual for the gap specification for your particular car. Spark plugs are gapped in thousandths of an inch. The gap for the spark plugs in one of my cars, for example, is 35 thousandths of an inch. This would be written in a shop manual or anywhere else an electrode gap specification is given as .035 in.

Not all spark plugs are gapped at .035 inc., though. The gap between the electrodes of some plugs should be wider. That's why it's important to consult the car-maker's specifications.

To change the electrode gap, the side electrode is bent slightly. If the gap between the side electrode and the center electrode is supposed to be .035 gauge should just barely be able to pass between the electrodes.

If in doubt about how to use a spark-plug gauge, the person at the auto parts sales counter can quickly and easily demonstrate it for you.

When installing new plugs, check the gap of each plug. If it's correct, bend the side electrode until it is correct. Don't assume that because a spark plug is new it's gapped to your car-maker's specification. t

Which replacement plugs should you buy? Any brand-name plug is all right; the important thing is to buy the size and type of plug recommended by the maker of your car. As long as you do that, and replace the plugs at the intervals recommended by the car-makers, along with doing the other recommended maintenance, your car will last longer, run better and give you the most miles for your fuel dollar.