The Environmental Protection Agency set new standards for diesel exhaust fumes yesterday and gave Detroit an extra two years in which to meet them. Automakers immediately said they expect to have trouble anyway.

The new standards limit the amount of particulate matter, or soot, that diesel cars and light trucks can belch forth. Limits already exist for other kinds of diesel emissions, such as nitrogen oxides, and will be issued for heavy truck and bus diesel engines later this year.

One in every 20 cars is diesel-fueled now, but the number will be one in four or five by 1990, said EPA Adminstrator Douglas Costle. "Diesel is fuel-efficient and durable, but dirtier," he told a news conference. "These standards guarantee that diesels will be a clean part of the national fleet."

Preliminary research has shown diesel emissions to cause cancer in some laboratory animals, but results are still inconclusive and $10 million worth of studies are continuing, Costle said. Controls are warranted now because diesel soot is very fine and sticky and lodges in the deepest, most sensitive parts of the lung where it remains for extended periods, he said.

Costle linked diesel particles to emphysema, bronchitis and astham. "With large-scale switching to diesels, we will see an increase in these problems if [soot] is not controlled," he said. the new rules will add $12 to 1982 stickers prices and $138 to $164 to 1985 models, he estimated.

Because of incomplete burning of fuel, diesels produce 30 to 70 times as many particles per mile as gasoline-powered vehicles, Costle said. But their mileage if 25 to 30 percent better so that demand can be expected to grow. The new emission standard will reduce the particle count by 1990 to one quarter of what it would have been otherwise, cutting diesel emissions by 138,000 tons per years, Costle said.

Diesel cars and light trucks will be limited to no more than 0.6 gram of soot per mile by 1982. By 1985, the standard will drop to 0.2 gram per mile for cars and 0.26 grams per mile for light trucks, those weighing less than 8,500 pounds.

Industry spokesman said the technology for achieving 0.2 gram per mile does not now exist.

Frank Faraone of General Motors, which expects to be 13 percent diesel by 1985, said EPA's new timetable "could seriously jeopardize the use of diesel engines. . . without any basis for a significant effect on public health."

GM's R. T. Kingman said EPA's worry about lung problems was based on the premise that diesel exhaust would accomulate in urban canyons. Industry studies, he said, showed that traffic heat and air turbulence pushed the particles upward instead. "We think the timetable is much more stringent than it has to be to protect public health," he said.

A spokesman for Volkswagen, which expects its 1981 fleet to be 50 percent diesel in some models, said it needs a waiver on other emission controls if it is to meet the particulate regulation.

Technology to control nitrogen oxide emissions has the effect of increasing soot emissions, the spokesman said. "Meeting the 1985 standard will depend on developing new technogy to control both," he added.

EPA originally proposed the soot standards to go into effect in 1981 and 1983, but extended the period to allow the technical questions to be solved, Costle said. "We know we are pushing the technology," he said, "but the automakers have generally taken a 'cando' attitude."

The most promising approach so far is a scouring-pad type of metal mesh exhaust filter that catches the particles, which are periodically burned off automatically. Costle said he thought the so-called trap oxidizer could be made to last the life of the car without replacement, although no such filter is yet known.

Industry representatives testified at congressional hearings this week that all studies so far have found no relationship between diesel fumes and cancer, notably in a 25-year survey of London bus drivers. But Costle said EPA will reserve judgment until more facts are in.