The Environmental Protection Agency yesterday took the unprecedented step of ordering the Ford Motor Co. to stop manufacturing a line of new Ford Granadas and Mercury Monarchs that are polluting the air.

The agency made clear it is prepared to take similar action against other auto manufacturers if necessary.

Under the EPA order, Ford must stop manufacturing the 1977 Granadas and Monarchs unless their carburetors are adjusted to limit carbon monoxide emisions further. The agency ordered Ford to recall 54,000 cars.

The company said the adjustments are being made and that assembly lines will keep moving.

It was the first time EPA has intervened in the assembly-line production of automobiles. Previously, the agency set air pollution design standards for cars and assumed that the companies would follow them.

EPA's acting administrator, John R. Quarles Jr., predicted that the agency's assembly-line testing program, which began last month, "will have a major impact on the production processes of the entire auto industry, forcing the companies to adopt quality control programs."

Assembly-line testing, he said, "will reduce smog in our cities and provide assurance to the consumer that the pollution control equipment he purchases with his car actually meets the standards."

EPA ordered Ford to stop production of certain Granadas and Monarchs at its Wayne, Mich., and Mahwah, N.J., plants after discovering that the cars emitted, on the average, twice the allowable level of carbon monoxide.

The order affects about one-third of the Granadas an Monarchs under production - those with six-cylinder, 250-cubic-inch-displacement engines and automatic transmissions.

A Ford spokesman acknowledged "a quality problem in the carburators which resulted in high carbon monoxide emissions."

Under the recall program Ford is to notify purchasers of the 54,000 automobiles that they may have their carburators adjusted free of charge at local dealers.

The Granada and Monarch test was one of three undertaken in the first six weeks of EPA's assembly-line program. Chrysler's Fury and Monaco models and Pontiac LeMans and Grand Prix models met federal standards.

EPA plans to make 40 assembly-line tests a year, covering roughly 20 per cent of cars produced.

"We have long suspected that cars coming off the assembly line do not meet pollution control standards," Quarles said . "Until now, we had no concrete evidence."

Quarles said it is "significant and disturbing" that early testing has revealed "such substantial violations." Some of the cars tested emitted four times the permitted level of carbon monoxide, he said.

"This provides documentation of the concern that a significant number of cars coming off the assemlby lines may fail to meet auto emission standards," Quarles said.

Assembly-line testing is a major step in the government's auto pollution control effort, which began with the Clean Air Act of 1970. Limits were set on the amount of pollutants cars could mit. EPA undertook to certify new car designs each year to make sure they meet the limits.

"But a major gap in performance exists between the cars approved and certified and the cars actually on the highways," Quarels said. Last Year, EPA recalled 1.5 million cars already in use after investigations revealed they were overpolluting.

With assembly-line testing, EPA will be able to catch many of the poorly performing cars before they are sold, Quarles said.

Even if they pass assembly-line pollution tests, however, cars deteriorate later as a result of tampering and poor maintenance. A nationwide inspection and maintenance system to ensure that pollutants are controlled over the life of the car is "badly needed," Quarles said.