A scheduled maintenance, is different from a performance problem that will require additional testing to identify the cause (or causes) of the problem.

Only after the performance problem has been diagnosed should any parts be replaced.

TUNE-UP CHECKS. Any tune-up today should start with a battery of performance checks to base line or confirm the engine's overall condition. These should include: battery voltage (very important with all of today's onboard electronics); charging voltage; power balance or dynamic compression (to identify any mechanical problems such as leaky exhaust valves, worn rings, bad head gasket, bad cam, etc. that could adversely affect compression and engine performance); engine vacuum (to detect air leaks as well as exhaust restrictions).

Operation of the fuel feedback control loop (to confirm that the system goes into closed loop operation when the engine warms up) should also be tested.

Also scan for fault codes (to verify no fault codes are present, or to retrieve any codes that may be present so they can be diagnosed and eliminated.

Exhaust emissions (this should be a must in any area that has an emissions testing program to confirm the vehicle's ability to meet the applicable clean air standards, and to detect gross fuel, ignition or emission problems that require attention); idle speed (should be checked even if computer controlled to detect possible ISC motor problems); idle mixture (older carbureted engines only, but injector dwell can be checked on newer vehicles to confirm proper feedback fuel control.

Ignition timing (should be checked even if it is not adjustable to detect possible computer or sensor problems).

OPERATION OF THE EGR VALVE. In addition to these performance checks, hoses and belts should be visually inspected.

All fluids (oil, coolant, automatic transmission fluid, power steering fluid and brake fluid) should also be inspected to make sure all are at the proper level, and that the appearance and condition of each is acceptable.

There should be no sludge in the oil, the ATF should not smell like burnt toast, the coolant should have the proper concentration of antifreeze and not be full of rust or sediment, the brake fluid should be clear and not full of muck, etc.

WHAT TO REPLACE. If the tune-up checks find no major faults, the following items should be replaced for preventative maintenance: spark plugs (gapped to the correct specs, of course). Recommend long life plugs on applications where plug accessibility is difficult or where longer service life may be beneficial; rotor and/or distributor cap (if required); fuel filter; air filter; PCV valve and breather filter. Other parts should be replaced on an "as needed" basis (things like spark plug wires, belts, hoses, fluids, etc.).

Check and adjust (if required on older vehicles) ignition timing, idle speed and idle mixture as well as O2 sensor(s).

SPARK PLUGS. Spark plugs need to be changed periodically because the electrodes wear every time a plug fires. When high voltage current jumps from one electrode to another, it wears away a little metal from both electrodes.

After 30,000 miles of operation, the plug has fired 60 to 80 million times and wear has increased the distance between the electrodes.

At the same time, the nice sharp edges on the center electrode have become rounded and dull. All this increases the voltage required to jump the gap.

If the ignition system can't deliver, the plug may begin to misfire under load. Accumulated deposits on the plug tip may also be interfering with reliable ignition.

So by the time the average plug has seen 30,000 miles, it's getting close to the end of its service life.

Long-life plugs, on the other hand, don't wear as quickly as standard plugs.

The electrodes are made of tough platinum or gold-palladium alloys that resist erosion. Such plugs may go 60,000 to 100,000 miles under optimum conditions (no fouling).

Of course, no plug will last anywhere near its potential lifespan if an engine is burning oil, experiencing abnormal combustion such as detonation or preignition, or has a fouling problem.

NEW O2 SENSOR. Though most motorists don't even know what an oxygen sensor is, let alone that their engine may have one or more of these devices, the fact remains that sluggish O2 sensors cause a lot of driveability problems.

A recent EPA study found that 70 percent of all vehicles that fail an I/M 240 emissions test need a new O2 sensor. To prevent such woes, the O2 sensor can be replaced for preventative maintenance during a tune-up.

Unheated 1 or 2 wire wire O2 sensors on 1976 through early 1990s applications should be replaced for preventative maintenance every 30,000 to 50,000 miles.

Heated 3 and 4-wire O2 sensors on mid-1980s through mid-1990s applications should be changed every 60,000 miles. And on OBD II equipped vehicles (all Ô96 and newer), the recommended replacement interval is 100,000 miles.

The O2 sensor is the master switch in the fuel control feedback loop. The sensor monitors the amount of unburned oxygen in the exhaust and produces a voltage signal that varies from about 0.1 volts (lean) to 0.9 volts (rich).

The computer uses the O2 sensor's signal to constantly fine tune and flip-flop the fuel mixture so the catalytic converter can do its job and clean the exhaust.

If the O2 sensor circuit opens, shorts or goes out of range, it usually sets a fault code and illuminates the Check Engine or Malfunction Indicator Lamp. But many an O2 sensor that is badly degraded will continue to function well enough not to set a fault code but not well enough to prevent an increase in emissions and fuel consumption.

So the absence of a fault code or warning lamp doesn't mean the O2 sensor is doing its job.

Deterioration of the O2 sensor can be caused by a variety of substances that find their way into the exhaust (such as lead, silicone, sulfur, even oil ash) as well as environmental factors such as water, splash from road salt, oil and dirt.

A sluggish sensor may not allow the computer to flip-flop the fuel mixture fast enough to keep emissions within acceptable limits.

A dead sensor will causes the system to go back into open loop with a fixed, rich fuel mixture.

Fuel consumption and emissions go up, and the converter may suffer damage if it overheats.

The best way to check O2 sensor performance is with a digital oscilloscope.

A good sensor should produce an oscillating waveform that flip-flops from near minimum (0.1 to 0.2v) to near maximum (0.8 to 0.9v). O2 sensors in feedback carburetor applications have the slowest flip-flop rate (about once per second at 2,500 rpm), those in throttle body injection systems are somewhat faster (2 to 3 times per second at 2,500 rpm), while multiport injected applications are the fastest (5 to 7 times per second at 2,500 rpm).

When the mixture is made artificially rich by injecting some propane into the intake manifold, the sensor should respond almost immediately (within 100 milliseconds) and go to the maximum (0.9v) reading.

Likewise, making the mixture artificially lean by opening a vacuum line should cause the sensor's output to drop immediately to the minimum (0.1v) reading.

Other services. Another service that could and should be included in today's tune-up is cleaning the fuel injectors and intake system.

Fuel varnish deposits that form in injectors restrict the amount of fuel that's delivered with every squirt, which has a leaning effect on the air/fuel mixture. The result can be lean misfire and a general deterioration in engine performance and responsiveness.

Deposits can also build up on the backs of intake valves, causing cold hesitation problems in many engines. The cure is to clean the injectors and valves.

Cleaning should be recommended for any engine that is suffering a performance complaint or has more than 50,000 miles on the odometer.

Cleaning the throttle body can also help eliminate idle and stalling problems that plague many of today's engines.

Most experts still recommend changing the oil and filter 3,000 miles or three to six months. The oil change interval can be stretched out to reduce maintenance costs if a vehicle is driven under ideal conditions (no extremely hot or cold weather, no short trip, stop-and-go driving, no excessive idling, no extremely dusty road conditions, no trailer towing, no turbocharging).

A number of new cars and trucks now have "lifetime" fuel filters, most of which are located inside the fuel tank with the electric fuel pump.

Such a filter might go 100,000 miles. Then again, it might not. A couple of tanks of bad gas or some corrosion caused by accumulated moisture can cut short the life of any filter, even a so-called lifetime filter.

Sooner or later even a lifetime fuel filter will have to be replaced.

As for air filters, the service life depends more on environmental factors rather than time or mileage.

If a vehicle is driven on gravel roads, filter life may only be a few months or few thousand miles.

Brakes in city driving last- 20,000 to 30,000 miles as a more realistic figure.

The same goes for belts, hoses, the battery, water pump, exhaust system and many other parts.

No vehicle that's yet been built can even come close to going 100,000 miles without needing some type of maintenance or repair.