U.S. automakers were surprised this year by their strong sales and record profits, but neither analysts nor the companies think the performance will be repeated in 1989.

Sales momentum already is cooling and each of the automakers is seeing cost pressures that are likely to continue into the new year."We're jittery," said Chrysler Corp. director of sales operations planning Steve Torok about the sales outlook for coming months.

But automakers can look back at a 1988 that exceeded all expectations. Although Chrysler saw profits lag due to its effort to pick up market share, General Motors Corp. stunned Wall Street with a record third quarter in which its cost-cutting efforts finally paid off in bottom line improvements.

Analysts and automakers held little optimism for auto sales when the year began, thinking the October 1987 stock market crash would put a damper on the market. Few thought sales of cars and trucks would climb much above 15 million units.

But sales boomed in the first half of the year, fueled by constant incentive programs and stronger consumer confidence. Automakers will end the year with sales approaching 15.7 million units - the third best year in industry history.

"Basically, the story is that everyone overestimated the impact of the stock market crash, not only in terms of autos but in terms of the economy as a whole," said analyst Edward Sullivan of Wharton Econometrics Forecasting Associates Group.

Along with those sales came significant profit gains. Auto analysts expect combined U.S. auto profits to approach $11.5 billion this year, up 20 percent from 1987 profits of $9.5 billion.

The biggest gain is expected at GM, which analysts think will end the year with earnings of $4.5 billion to $4.7 billion, up from $3.5 billion last year.

Ford Motor Co. is expected to end 1988 with profits of $5 billion to $5.5 billion, up from $4.6 billion. Chrysler, which chose to concentrate on market share gains at the expense of profits, should earn $1.1 billion this year, compared with $1.2 billion last year.

Although Chrysler is expected to see its profits grow in 1989 to about $1.3 billion, GM and Ford each are expected to see declines due to greater cost pressures and somewhat slower sales.

Analysts see profits of $4 billion at GM in 1989 and $4.6 billion for Ford next year, with combined industry earnings at about $10 billion.

In car sales, Ford posted the biggest gains in market share and volume this year. The automaker saw two new models - the sporty Ford Probe and luxury Lincoln Continental - become instant sell-outs.

Ford on Dec. 26 officially launched its restyled Ford Thunderbird and Cougar models, and initial dealer orders indicate another success for the new year. Another hit is the recently introduced Taurus SHO, which has a powerful high-output engine.

GM, which had targeted 1988 as the year it would regain its lost market share, instead lost ground to its competitors and will end the year with sales just even with 1988 levels.

GM thought its GM-10 coupes would help it win back market share, but sales of the two-door models fell far below its expectations. The automaker now claims the four-door versions of the cars, on tap for 1989, will boost its share.

Unlike GM, Chrysler was successful in its all-out bid to gain market share. Boosting Chrysler's fortunes were strong sales by its Jeep unit, which it acquired in 1987 when it purchased American Motors Corp.

But U.S. automakers could not ignore the growing threat posed by Japanese automakers building so-called "transplant" models in the United States.

Toyota Motor Co. opened its first U.S. plant during 1988, and Chrysler's joint venture with Mitsubishi Motors Corp., Diamond-Star Motors, built its first cars. Honda Motor Co. began construction on a second U.S. plant, to be completed in 1989, while Mazda Motor Corp.'s Flat Rock, Mich., plant began to approach full production.

Each of these automakers will gain production capacity during 1989 and analysts estimate transplants, which took just 7 percent of the U.S. market in 1986, will account for almost a quarter by 1990.

In addition, Japanese automakers are set in 1989 to launch luxury import models that will compete head-on with European automakers.