As part of its effort to restore economic peace with the United States, the Japanese government is urging automobile manufacturers to limit their exports to the United States.

So far, the government has not issued any explicit order fixing the number of cars and trucks Japan should sell in the American market.

But industry sources said this week government officials have given them what is called "vague administrative guidance" to restrain sales voluntarily as part of the national campaign to reduce Japan's trade surplus with the United States.

At least one major Japanese manufacturer, Toyota, believes the guidance is unnecessary and said it has no plans to curb its exports artificially.

A spokesman said Japanese exports to the United States will automatically level off this year, partly because the increased value of the yen makes Japanese cars more expensive and partly because of greater competition from American small-car manufacturers.

The Japanese government, however, is worried that if the current trend continues it will cause a new round of trade conflicts such as those last year involving television sets and finished stell products.

The government believes that if foreign imports exceed 20 per cent of the total American market the trouble will start. In January, automobile imports to the United States for the first time went over the so-called 20 percent "warning line," hitting 20.6 percent about two-thirds of those imports were from Japan.

Speculation that this would spark a renewed trade conflict cropped up in the Japanese parliament late last month and Japan's foreign trade minister, Nobuhiko Ushiba, declared, "Automobile industry cicles had better export cars with caution." Breaking of the 20 percent barrier, he warned, would create pressures for import restrictions in the United States.

Thatt was followed by what a Toyota official described last week as a "vague administrative guidance" from the powerful ministry of international trade and industry, whose advice to big corporations frequently is heeded promptly.