American trade representativ es today cautioned Japan not to become complacent because of a favorable U.S. government ruling on its high-volume automobile exports.

They warned Japanese officials that there is still significant political pressure within the United States to force restrictions on auto imports, and urged them to take steps that might dilute that pressure.

Two days of economic talks between high-ranking officials of both sides ended with no new agreements but none had been expected, in part because the U.S. leaders are serving out a lame-duck administration and Japan is waiting to see how their successors approacch trade issues.

Robert Hormats, deputy U.S. trade representative, said the Japanese had been told that support is growing in Congress for a bill authorizing the president to negotiate what is called an orderly marketing agreement to restrict the volume of Japanese cars imported into the United States.

Hormats and Under Secretary of State Richard Cooper reported that they had urged the government of Japan to encourage two measures by private Japanese companies that might lessen the pressure.

One would promise to increase significantly the import into Japan of U.S.-made automobile parts. The other would encourage Japanese auto investments in the United States, principally through tie-ups between major auto producers in both countries.

Their warnings followed last week's ruling by the U.S. International Trade Commission that Japanese imports are not the major cause of the American industry's sales slump that has caused widespread unemployment. The decision came as a great relief to Japanese manufacturers, but Japan's principal representative, Kyoaki Kikuchi, deputy minister for foreign affairs, acknowledged that the favorable U.S. ruling did not mean a "political solution" to the auto import question has been reached.

According to one source, the government here is preparing to respond favorably to requests that Japanese companies invest more capital in the U.S. auto industry and increase purchases of American-made parts, few of which are imported into this country.

There is no assurance, however, that the Japanese companies would follow their government's lead on those two matters. So far, the major auto producers here have hoped that political pressures would subside gradually in the United States because the Japanese share of the U.S. car market has declined in recent months from unprecedented heights reached last summer.

Hormats said the two countries seem to be "on the threshold" of an agreement that would give U.S. tobacco companies more access to the Japanese market. Those companies now share only about 1 percent of that market.

Hormats said the issue is an important symbolic one because the market is controlled by a Japanese tobacco monopoly corporation over which the government holds direct control.