In the textbooks that economists live by, it is all supposed to be neatly predictable. When Japan's yen goes up, the country's exports, becoming more expensive, go down. Then the world stops worrying about that terrible trade surplus.

The problem is that it has not happened. The Japanese yen has appreciated in value nearly 25 percent and still those Toyotas, Datsuns, Seiko watches, television sets, desk calculators and other items keep pouring out to the world, some at record rates.

The trend may reverse itself this year, but for the moment Japanese officials who wanted to appease the world by trimming the trade surplus are embarrassed and slightly baffled.

"It's a very serious situation," said one prominent trade official last week. "Our exports go on rising relentlessly. People are rushing to export. It's quite different from classical theory."

He and other officials now estimate no substantial change in the high-export trend until this fall or later. They no longer expect to hit their much-talked-about goal of reducing the current account surplus to $6 billion this fiscal year.

Although that prospect could provoke a nasty new arugment with the United States and Europe, Japan has just about decided not to take the drastic step of clamping mandatory export controls on big companies.

Since the early months of 1977, the Japanese yen has risen in value against the dollar by 25 percent - going from 292 yen to the dollar to the current 218. Conventional theory holds that such a large appreciation should have automatically depressed, or at least restrained, Japan's exports. All those watches and television sets should have become too expensive to remain competitive in foreign markets. The American buyer should have switched from the Toyota to Detroit's latest compact.

But the actual trend was different. Exports have continued to increase, disproving one projection after another. In the fiscal year just ended, Japan's total exports were at least $5 billion more than anticipated a year ago by economic planners - despite Japan's voluntary curbs on exports of television sets during 1977.

Some government officials and private economists say this is merely a short-run phenomenon that will fade out. Exporters are rushing to sell their goods abroad quickly before the yen goes even higher, they contend, and in the meantime are benefitting from the lower costs of imported rawmaterials.

The trade surplus grows even bigger. "In the short term," says James C. Abegglen of the Boston Consulting Group, "everything gets worse."

Kiichi Miyazawa, director general of the economic planning agency, agrees. Big corporations in the early months this year pushed hard to sell abroad, fearing still a further rise in the yen, he said in an interview. Those who sell imported goods here, on the other hand, held off buying, waiting for the still-cheaper dollar to [TEXT OMITTED FROM SOURCE]

He predicts the trend will play out early this fall and hopes that the United States and Europe will not get too impatient in the meantime.

Other economists point out there are peculiarities of Japan's economy that prolong the high-export phenomenon. For one thing, Japan's major export items have become remarkably appealing around the world and they keep on selling well regardless of price increases.

Its television sets are renowned for low service costs and its cars for low fuel costs.

Seiko watches and several lines of optical equipment are in demand at almost any price. Toyota has increased its prices six times in the past seven months, but its March sales in the United States were 4 percent higher than in the same month of 1977. Sales of Datsuns increased 14 percent. If the American wants to buy a video tape recorder, he buys a Japanese make because there are no others.

Another economic fact of life is that the typical Japanese corporation continues to export even at low profits or at a loss. Japan's domestic demand is stagnant and there is nowhere else for Japan to go with its goods.

Because of long-standing commitments of life-time employment for its workers, the Japanese company does not trim payrolls; it keeps on producing and exporting so it can keep those commitments.

At one time, the government seriously considered mandatory export controls to curb the biggest sellers - such as car manufacturers - whose foreign sales might disrupt American and Europeon markets. The idea is still being debated, but officials who once favored it have cooled, fearing it would ruin the economy. It was strongly opposed by the Ministry of International Trade and Industry.

Miyazawa, who once leaned toward invoking the export control law on some items, said he has abandoned the idea. If foreign leaders realized Japan was willing to curb one line of export products, there would be pressures for it to restrain others, too, he said.

American congressmen, he said, would start saying, "Look, Japan is doing this on automobiles, why can't they do some on other items. This will start to be a Christmas tree and it might initiate a worldwide protectionist movement.