One piece of advice that is being recycled these days is to tune up the car to reduce pollution.

Traditionally, that meant checking the points and the mechanical timing of the sparkplug firing and adjusting the carburetor. But those remedies are as outdated as whale-oil lanterns. Few cars built since 1975 have points and their timing is electronic. Since 1985, carburetors have largely been supplanted by fuel injection.

So what is there left to do? Quite a lot, mechanics and environmentalists say, although they differ on just what.

The best way to cut pollution from a car, environmentalists say, is to leave it in the driveway, because even a properly functioning automobile emits 0.4 to 1.3 pounds of carbon dioxide a mile. Carbon dioxide is harmless to humans, but adds to the atmosphere's inventory of gases that trap heat and are believed to contribute to global warming.

A bus or train uses half as much energy per passenger-mile, or less, and cuts carbon monoxide and hydrocarbon pollution by substantially more than half. Better yet, use a bicycle.

For those who have to drive, and much of the country does, a well-maintained car can still get 9 percent more mileage on a gallon of gasoline than a poorly maintained one. That means 9 percent less carbon dioxide, as well as less nitrogen oxide and hydrocarbons, which cause smog, and carbon monoxide.

Oil companies have begun marketing gasolines designed to burn more cleanly, although the extent of changes in the gas varies widely. While oil companies and car manufacturers struggle toward cleaner car-fuel combinations, engineers say there are steps motorists can take now to decrease pollution, save money and keep a car running in top form. Some of the advice is the same as it always was: Keep the tires properly inflated, to cut rolling resistance.

Change the oil on schedule, because old oil gets thin, and will seep around the piston rings into the combustion chamber, and burn, causing pollution. If the oil gets old enough, it will thicken, increasing friction. Some advice is new: Don't spill gasoline at the self-serve pump, because unburned gasoline releases chemicals called volatile organic compounds, which react in sunlight to form ground-level ozone, or smog. When the pump shuts off automatically, don't ''top off.'' Other steps can help, too, and are still referred to as a tuneup. Here are three things to check on: air filters, fuel injection systems and spark plugs. George C. Giek, managing director for automotive engineering at the American Automobile Association, said modern cars often run ''dirty'' because the engines are meant to run on a very precise mixture, usually 15 parts air to one part fuel. But if the air filter becomes clogged, the ratio can fall to 10 to 1 or even 7 to 1, he said.

''Car owners believe the air filter gets clogged up more rapidly in desert areas, and this is true,'' he said. ''But it also happens in areas where a lot of snow falls. Snow-removal chemicals get ground up into dust, and they tend to very rapidly clog up air filters.''

Modern cars have instruments that measure the exhaust and feed the information to microprocessors, which issue commands to the fuel injection system to compensate. But there are limits to the adjustments the car can make.

Many oil companies now advertise that their fuel contains detergents to keep fuel injectors clean. More than one detergent is needed because each is designed to work at a different temperature, in a different part of the engine. California may soon require those ingredients in gasoline, and if it does, other states are likely to follow.

Another source of trouble is a cylinder that skips, pushing its fuel out unburned. Spark plugs, a component of tuneups in the past, are less prone to cause skipping in modern cars than in old ones, because new plugs today last for 40,000 miles instead of 6,000. But the wires that carry the electricity to them are more vulnerable to failure now, because designers, trying to make cars quieter, have heavily insulated the engine compartments. The result is to raise under-hood temperatures as high as 400 degrees, an environment that also poses challenges for hoses and belts.

The heart of the anti-emissions system, the catalytic converter, generally functions without maintenance, as long as unleaded gasoline is introduced. The converter loses effectiveness as the car passes 50,000 miles, but states generally do not require car owners to repair them. Future models, however, are likely to carry converters that last longer.