Ford Motor Co., helped by lower operating costs and smaller currency exchange losses, posted a second-quarter profit of $205 million yesterday--the company's first profit this year and the best quarter since the third quarter of 1979.

The bulk of Ford's profits came from overseas sales. Ford's U.S. earnings last quarter were only $23 million due to "particularly weak" domestic sales, the company said.

The $205 million profit($1.70 a share) compared with $60.2 million (50 cents) in the second quarter of 1981 and was the best since third-quarter 1979's $512.4 million profit.

Ford finished the half with a net loss of $96.6 million, largely because of a $302 million operating loss in the first quarter. The company lost $379.3 million in the first half of 1981.

Ford's second-quarter success puts total earnings for the period at $803.2 million for the nation's four top automakers--General Motors Corp., Ford, Chrysler Corp. and American Motors Corp. The companies earned $575.8 million, or 40 percent less, a year earlier. Volkswagen of America, the fifth domestic producer of cars and trucks, does not have to disclose quarterly earnings because it is considered a subsidiary of the parent VW in Germany.

Most of Ford's after-tax profits in the second quarter, $190 million, came from its European operations. Overseas operations yielded $6 million in the second quarter last year.

The $23 million U.S. earnings compared with a profit of $82 million a year ago.

In Canada, Ford's net losses were $8 million for the second quarter of 1982, compared with a loss of $28 million in the second quarter last year. Chairman Philip Caldwell and Ford president Donald E. Petersen said the continuing Canadian losses "emphasize the need for further efforts to improve the cost structure of Ford's Canadian operations." In short, the company officials mean they will have to be as aggressive in trimming Canadian costs as they were in streamlining their U.S. operations, where Ford has dropped about 25 percent of its white-collar work force.

Worldwide dollar sales of Ford products yielded $10.9 billion in the second quarter, down 3 percent from $11.3 billion a year ago. For the half, sales were $19.8 billion, compared with $20.7 billion last year.

U.S. Steel Corp., citing the "severe depression" in the steel industry, said yesterday its second-quarter profit plunged to $4.3 million from $167.6 million a year earlier.

U.S. Steel, the nation's largest steelmaker, said its latest profit came to 5 cents a share, compared with $1.89 a share a year ago. Second-quarter sales, however, rose to $5.1 billion from $3.79 billion.

In the first half of 1982, U.S. Steel's profit fell to $84.2 million, (92 cents a share) from $438.5 million ($4.95) in the comparable 1981 period. Six-month sales rose to $10.1 billion from $7.2 billion.

U.S. Steel's profit was helped by an increase in the operating earnings of Marathon Oil Co., which U.S. Steel acquired in a $6.8 billion deal concluded in March.

U.S. Steel posted a 71 percent decline in earnings in the 1982 first quarter to $79.9 million, down from the $270.9 million for the 1981 first quarter.

Xerox Corp. had a 10 percent drop in total earnings on a 36 percent decline in profits on continuing operations in the second quarter.

Net income was $161.4 million ($1.91) on revenues of $2.11 billion compared with $179.7 million ($2.13) a year ago on revenues of 2.19 billion. Excluding a $45 million gain on the sale of WUI Inc. and profit from WUI, net income in the latest quarter was $112.6 million ($1.33).

First half net was down 16 percent at $270.9 million ($3.20) on revenues of $4.1 billion compared with $322.4 million ($3.82) a year ago on revenues of $4.2 billion. Net on continuing operations was $2.54 a share, down 32 percent from a year earlier.

Eastern and Northwest airlines posted second-quarter and first-half losses, both citing low traffic and industrywide discount fares.

Eastern, the largest passenger carrier in the West, and third-largest U.S. carrier by revenue last year, said it had a net loss of $3 million in the second quarter, compared with a $4.6 million loss in the same period a year ago. Revenue was $967.3 million, down from $970.8 million.

Northwest, whose 3,000-member machinists union struck for three and a half weeks during the second quarter, reported an April-June loss of $1.4 million compared with a profit a year ago of $3.4 million (16 cents).

Revenue was $414 million, down from $470 million.

In the first half, Northwest lost $19.4 million on revenue of $829.5 million, compared with a first-half loss a year ago of $4 million on revenue of $889.7 million.

Eastern said it had a net loss of $54.5 million in the first half on revenue of $1.88 billion, compared with a loss of $515,000 in the first half last year on revenue of $1.93 billion.

Tenneco Inc., the diversified natural gas company, reported its second-quarter operating earnings rose 14.2 percent on the sale of Canadian oil and gas properties.

In the second quarter Tenneco had a profit of $232 million ($1.44) from continuing operations, up from $203 million ($1.50) in the same period last year. Revenues were $3.7 billion in both quarters.

In the latest period Tenneco's sale of Canadian oil and gas producing properties resulted in a gain of 52 cents a share, which was partially offset by the loss of 22 cents a share from discontinued operations.

Tenneco's net income for the 1982 quarter was $204 million ($1.44).

Tenneco said oil exploration and production, natural gas pipelines, shipbuilding and automotive components showed improvement during the quarter.

For the first half of 1982 Tenneco had net earnings of $333 million ($2.32), down 16.1 percent from $397 million ($3.01) a year earlier. Revenues rose to $7.54 billion from $7.51 billion.