The Environmental Protection Agency said yesterday that about 80 percent of the nation's cars manufactured since 1971 are illegally polluting the air.

Although automobile companies have been required to install auto emission controls for the last eight years, widespread maladjustment of engine settings, deliberate tampering and illegal use of leaded fuel have rendered them ineffective, the agency acknowledged.

The findings, contained in a general Accounting Office report released by Sen. Edmund Muskie (D-Maine), confirm what government officials have suspected for years -- that without extensive post-assembly-line supervision, regulation and inspection, auto emissions standards alone won't clean up the air.

According to the report, a quarter of the cars coming off assembly lines don't meet federal pollution standards, more than half exceed the limits within a year, and the failure rate increases with age.

The report recommends that annual inspection programs be set up in virtually all major American cities -- a requirement of the 1970 Clean Air Act that has yet to be implemented.

"The public has not generally accepted the need for I&M [inspection and maintenance] programs, primarily because car ownners bear the cost to correct cars failing inspection," it noted.

Benjamin Jackson, head of Epa/'s auto enforcement program, said the agency "generally agrees with the report and we have programs inn place to deal with the problems."

However, EPA Administrator Douglas Costle is expected today to relax the general smog standard, thus exempting about 18 cities from haveing to adopt inspection programs.

The GAO report charged that EPA's program to certify that cars meet pollution standards does not account for "real would driving conditions" such as very hot or cold temperature, poor roads and deterioration of engine parts.

Also, it said that the agency's assembly-line testing program covers "only a small fraction of the 8,700 car model configurations." And, while EPA has recalled 12 million cars for emission defects since 1972, many owners simply don't return their cars for correction.

Under the law, cars must be designed to meet federal emission standards for five years or 50,000 miles. While about 80 percent are not meeting those standards, cars today still show a measurable improvement over their 1960s ancestors, Jackson said.

He estimated that the 80 million cars with emission controls spew out 60 to 80 percent less pollution. A car can fail emissions standards by violating carbon monoxide levels, but still meet hydrocarbon and nitrogen oxide regulations, he added.

The agency has begun prosecuting people for tampering. Auto dealers can be fined $10,000 and garge mechanics $2,500 for illegally misadjusting pollution controls.

Jackson said EPA plans an extensive public information program to inform people that tampering with emission devices does not improve fuel economy. "That's a common myth," he said.

Inspection and maintenance programs, however, would help clean up most of the remaining auto pollution problem, according to the report. So far only a few scattered jurisdictions, including New Jersey, Phonix, Cincinnati and Portland, have adopted them.