A tune-up can mean different things, depending on the engine. But the one thing all tune-ups have in common is new spark plugs. Nowadays, the recommended interval for a spark plug change ranges from 25,000 to 100,000 miles. Your owner’s manual spells out exactly what the manufacturer expects for your particular model and its engine.

On newer cars—those with fuel injection and electronic ignitions—a complete tune-up also includes checking the ignition system (using a diagnostic tool), the engine knock sensor, the positive crankcase ventilation (PCV) system and the oxygen sensor, replacing or repairing
components as necessary. The technician will analyze the exhaust to check for proper combustion. A tune-up should also include a check of the engine timing and idle speed. On most newer cars, however, a computer automatically manages the base timing and idle; a technician can ensure that they’re within specs, but can’t necessarily adjust them.

On older cars—those with distributors— a tune-up involves not only changing the spark plugs, but also checking and adjusting the distributor cap, ignition rotor and other components. And on very old cars—those with breaker-point distributors, rather than lectronic ignition controls—a tune-up includes replacing and adjusting the points as well.

A well-tuned engine runs at peak efficiency, giving you the best gas mileage and performance possible. You should have a complete tune-up at least as often as specified in your owner’s manual—more frequently if you do a lot of stop-and-go driving or heavy towing.

Think of the tune-up as part of regular reventive maintenance, not as a cure-all for whatever ails your engine. If you experience a specific problem between tune-ups, tell your technician. Describe the symptoms and the circumstances in which they occur in as much detail as possible. That way, you can head off unnecessary expense and won’t drive off disappointed.