The current economic expansion will be 5 years old this fall. It will shortly become the second longest since World War II, having survived a huge rise in the value of the dollar, near depression in the agricultural sector and a collapse of the nation's oil and gas drilling industries. The recovery remains quite unlike any of its predecessors.

The rise in the value of the dollar produced a flood of imports and badly hurt American exports. The resulting increase in the nation's trade deficit clobbered some manufacturing industries. But big increases in consumer spending and housing construction helped offset the shock to the economy.

And even the near depression in agriculture and a collapse of oil and gas drilling in the wake of falling energy prices were not enough to bring on a recession.

Economic forecasters still expect an economic slump, but they keep pushing the date into the future. None of the economic strains that usually bring on a downturn, such as excessive business inventories, tight labor and product markets and steadily accelerating inflation, have emerged.

Later this week the Commerce Department will release its preliminary estimate for economic growth in the second quarter, as well as revised estimates of the gross national product for 1985 and 1986.

Most economists now expect the second-quarter GNP, adjusted for inflation, to be up much less than the first quarter's 4.8 percent rate. In fact, a significant number of prominent forecasters predict the second-quarter GNP growth rate will be 1.5 percent or less.

The first-quarter surge was primarily the result of a big jump in business inventories and a more modest improvement in the enormous trade deficit. A drop in the trade deficit means that more of the nation's demand for goods and services is being satisfied by American producers than by producers abroad.

The trade gains clearly continued in the second quarter, but business added to its inventories by much less than it did in the first three months of the year.

Meanwhile, consumer spending, business investment and government spending, all of which fell somewhat in the first quarter, rose in the second quarter, analysts say. The problem was that the drop in business inventories was so large that it almost offset the other gains.

But that has often been the story of this long-running expansion. Small inventory swings have produced sharp changes in real GNP growth rates from quarter to quarter. But the fact that no mass of unwanted inventories has ever accumulated has been a major reason the expansion has continued so long with moderate growth rates.

With the latest miniswing in business inventories over, the dominant forces controlling the pace of the expansion are the continued improvement in the trade deficit and a slowing of consumer spending. Rising mortgage interest rates are expected to cause small declines in new home construction, while business investment in new plants and equipment likely will rise only slightly in real terms, according to several forecasters.

Adding up the pluses and minuses, forecasters generally come up with an economy growing at about a 2.5 percent to 3 percent rate in the second half of the year -- just about the same, or a little better than, in the first half.

Perhaps the biggest uncertainty -- now that the trend toward lower trade deficits has been established -- is just how much consumers will cut back. Households are being pinched by a big load of debt and incomes that are rising no faster than prices. On the other hand, their balance sheets are still in good shape because the value of many of their investments in homes and the stock market have gone up as fast as their debts.

"Consumer spending is losing steam, which will limit the pace of economic expansion over the coming quarters," said economist Charles Lieberman of Manufacturers Hanover Trust Co. "The consumer is not in trouble. However, growth in consumer outlays over the past three years was well above sustainable levels. As a result, the expected slowdown in the growth of household purchases is now under way, offsetting the gains coming from a gradually improving trade sector and creating a significant drag on the overall growth rate of the economy."

Lieberman notes that from the middle of 1984 -- when the first burst of economic growth that followed the 1981-82 recession slowed -- until the end of last year, increases in consumer spending more than accounted for the rise in demand in the economy. In other words, taken altogether, the remaining parts of economy -- business investment, trade and government spending -- did not grow at all.

"However," Lieberman said, "this increase in {consumer} spending was not matched by a comparable rise in income." The result was a drop in household saving rates.

Now the rates of change in spending and incomes are probably converging. Of course, some burst of economic activity could boost real incomes but, more likely, spending will slow until it is rising in line with increases in incomes, Lieberman said.

Another aspect of slower consumer spending growth is that, unless GNP growth accelerates sharply, demand in some part of the economy must go up less rapidly to make room for the improvement in the trade deficit. Simply put, goods and services that are exported cannot also be consumed or invested here.

"The large trade deficits, the capital inflows needed to finance them and the mounting external debt {the debt the United States owes to foreigners} means that the United States cannot continue to consume more than it produces," said the summary of the latest forecast from Wharton Economics. "Quite the contrary, the United States will soon have to produce more than it consumes to pay off its debts. This means that consumer spending will have to grow more slowly."

The Wharton forecast -- which includes a very weak third quarter and then a jump in the GNP to a 4 percent growth rate in the final three months of the year -- projects an annual increase in real consumer spending of 2.3 percent between 1987 and 1989, compared to a 4.1 percent annual increase between 1984 and 1987.

But while that slower pace in consumer spending normally would put a lid on GNP growth, it may not in this case. Continued improvement in the trade deficit will require steady increases in production of manufactured goods for export or to supply domestic purchasers who have switched from imported goods to U.S. goods as a result of rising import prices. The big decline in the value of the dollar since the beginning of 1985 is responsible for the increase in the relative price of imports.

However, to meet this new source of demand, American manufacturers may have to step up investment so they can have the necessary production capacity.

"This excess of production over consumption will require substantial amounts of new investment," said the Wharton summary. "New plant and equipment spending and the turnaround in real net exports will sustain growth in the medium to long term. Thus, there will be a secular shift away from consumption toward investment."

In the short run, some analysts expect the slower economic growth during the middle of this year to help cool off inflation. Spurred by rapidly rising food and energy prices, both consumer and producer price indexes had taken off earlier this year.

However, both the consumer price index and the producer price index for finished goods rose only 0.3 percent in May, and PPI went up only 0.2 percent in June. The June CPI will be released next week.

Some analysts, such as Allen Sinai, chief economist of Shearson Lehman Bros. Inc., believe the CPI will continue to rise at a 4.5 percent to 5 percent rate for the next couple of years. Others, including Joel Popkin of Joel Popkin and Co., a Washington consulting firm, expect more modest price increases. Over the next 12 months, Popkin said, the CPI will go up 3.7 percent and at only about a 4 percent rate in the 12 months following that.

"In early 1988, inflation pressures will build again, this time reflecting a pickup in wages," said Popkin. "Private industry wage and salary increases have decelerated steadily from a high of 9 percent in 1980 to 3.1 percent in 1986. This year they will run at about a 3.5 percent rate, up slightly from 1986 but less than this year's inflation rate.

"Thus, 1987 is likely to produce the first significant decline in real wages since 1980. With new minimum wage legislation likely to pass Congress this year, a 15 percent boost in minimum wages next January will galvanize pressure for wage boosts next year... . By the second half of next year, inflation will be approaching a 5 percent annual rate," Popkin estimated.

Since wages represent a major portion of business costs, inflation rates likely won't go up very much, other than temporarily, until wage increases accelerate. Businesses have been testing the price waters whenever they could to see if they can increase their profit margins. To some extent, they have been successful: Both higher prices and continued efforts at cost control are producing better margins and rising overall profits.

Higher prices for imported goods have given some competing U.S. manufacturers more room to increase their selling prices without losing sales, but prices for imports other than automobiles have gone up less than many analysts had expected. Many foreign producers, eager not to lose market share, have accepted lower profits in the wake of the dollar's decline rather than raise their prices in dollar terms.

So far, the Federal Reserve has moved to counter the stronger inflation fears, generated earlier this year by the rising price indexes, only when financial markets seemed to demand it. The Fed tightened monetary policy in March and April both to emphasize its anti-inflation attitude and to support a falling dollar. With the dollar more stable in recent weeks and some key short-term interest rates falling, some financial analysts last week said they believe the Fed has reversed some or all of its spring tightening.

Fed officials have indicated they are not that worried by higher import prices because that is the only way the huge trade imbalance will ever be corrected. At the same time, they have warned that an acceleration in the rate of wage increases could cause them to clamp down to head off a new inflation spiral.

sw If Popkin's view of wages is correct, then such Fed action is a year or more in the future. And that suggests that the economic expansion will run that long, or longer, before the familiar signs of a recession begin to accumulate.

Something always could go wrong, and the quarter-to-quarter course of the economy -- at least as reflected by the GNP estimates -- could be just as bumpy as it has been for the past three years.

But for now, this highly unusual expansion seems set to continue.sw