Washington area automobiles are going downhill, mainly because more motorists are pumping their own gasoline and neglecting routine maintenance, the American Automobile Association said yesterday.

"Cars here are definitely deteriorating; there's no doubt about it," said AAA spokesman Tom Crosby.

He said the deterioration can lead to bigger repair bills, inconvenience for motorists and greater risk of accidents.

For example, he said, tires that are improperly inflated can cause a loss in driver control and loss of coolant can lead to a cracked engine block, especially during colder months.

Motorists can combat such problems by making sure that basic maintenance checks are performed regularly on their cars, Crosby said. AAA recommends that motorists either do the work themselves or buy every fourth fill-up from a full-service station that provides basic maintenance.

At a news conference, the AAA announced a campaign with area service station operators to do basic maintenance checks when motorists use full-service gasoline pumps.

Participating stations will display bright blue and white posters that pledge the three-minute AAA recommended maintenance check, which includes a check of motor oil, radiator coolant, belts and hoses, battery fluid, tire pressure and tire condition, and cleaning of front and rear windows.

Of the 3,015 local vehicles checked by the AAA computerized Diagnostic Service in 1984, "more than half the cars had improperly inflated or badly worn tires, one car in three had defective lights, one in four had low coolant fluid or a neglected battery and one vehicle in five had a faulty drive belt," Crosby said.

Checked cars were found to be in worse condition than the previous year, when 2,689 vehicles were tested by AAA. For instance, 58 percent of the cars checked in 1984 had tire faults, compared to 33 percent the previous year; 32 percent had defective lights in 1984, compared to 26 percent previously; 22 percent had drive belt problems last year compared to 15 percent before; 23 percent had batteries with corroded terminals or low water, compared to 12 percent before, and 25 percent had coolant problems, compared to 12 percent in 1983. The oil level was too low in about 19 percent of the cars tested in both 1983 and 1984.

Crosby said that only about 9 percent of domestic automobiles tested in 1984 had no maintenance problems, another indication of the deterioration of automobiles from the previous year when about 25 percent of those checked were without maintenance problems.

Meantime, more and more people are buying self-service gasoline. "About 76 percent of all drivers now pump their own gas," Crosby said, "and the trend for that is up."