Just before his mysterious disappearance after a fiery truck crash earlier this month, Roman M. Leimer seemed to be on top of the world, telling friends that his wine business was flourishing and talking of plans for a wine festival in the spring.

But his ebullient personality apparently masked serious financial problems that had left the Austrian-born Leimer, an Exxon truck driver and wine importer, with the world caving in on him, his fledgling wine business threatened by debts and mounting lawsuits.

"He seemed to speak larger than he could produce," said Bill Wagner, owner of Wagner Vineyards in Lodi, N.Y., who said his small winery stopped doing business with Leimer because of slow payments. "He seemed to be very optimistic. He was biting off more than he could handle."

Leimer was presumed dead when an Exxon gasoline tanker he was driving was found ablaze on I-66 near Front Royal, Va., on Jan. 13. But last week a scientist called into the case by Virginia medical authorities said that the remains found in the truck were those of a pig, not a human being.

Leimer's wife and three children held a memorial service a few days after the fire and, according to Mrs. Leimer's lawyer, Richard Paugh, "she is convinced he is dead." The family has remained silent since the scientist's findings were made public and since a Virginia prosecutor ordered an arson probe of the blaze.

Authorities said they do not know whether Leimer is dead or alive and that they have not ruled out anything, including a possible case of homicide, arson, fraud or missing person.

The mystery that has surrounded Leimer's disappearance for the past week adds one more feature to a curious portrait that friends paint of Leimer, a short, heavy-set man who looked older than his 39 years.

Leimer was known to associates and friends as a man who "ate well and drank well," a man of many tastes and accomplishments.

To one he was an avid waterskier; to another he was a former member of the Austrian national soccer team; a newspaper article last year described him as a former upholsterer. Leimer told some acquaintances that he had been a mathematics professor in Austria, and his business card, one associate says, bore the title, "Dr."

A number of his business associates said they were surprised to learn that Leimer's principal occupation was driving a truck for Exxon.

Leimer told acquaintances that he was close to retirement at Exxon and, friends said, he became increasingly consumed with promoting his two-year-old wine business, Romax Inc., in Jessup, Md., which distributed a variety of Austrian and American vintages.

He founded an Austrian Music and Wine Society that held dinners in Baltimore and Washington. But friends said he lost a bid several years ago to become president of the German Saengerbund, a music society, because members feared he would use the position to advance his business.

Last summer Leimer joined with Watergate Wine and Beverage for a lavish wine festival held at the fashionable Watergate Mall, an event he thought was a great success.

Hoping to expand his wine list, he also spent several weeks visiting New England wineries in search of American wines to sell.

Last fall, he tried to promote his wines in conjunction with a Capital Centre show of Austrian Lippizaner horses, said a spokesman for the Austrian embassy.

Friends said Leimer told them he helped organize the show and the presentation of one Lippizaner to President Reagan by the Austrian Chamber of Commerce.

"He seemed to be putting himself under more and more pressure in this last year," said a business associate who asked not to be named. "He tried to do more things than anyone could do. But he seemed to be happy most of the time."

Leimer launched his business in September 1980 by signing a $46,000 loan agreement, drawn up on Leimer's personal stationery, with a friend who was an Exxon dealer, according to court records.

But about a year later Leimer's business partner sued him, alleging he had failed to live up to their agreement.

Leimer denied the charges and the suit was dismissed in late 1981 after he signed another agreement promising to repay the loan using his Burtonsville home as collateral, court records show.

Last September, however, Leimer defaulted on the loan and a new suit was filed.

In December, according to court records, he was notified that he had to pay $44,000 on the loan.

He was allowed one month to give the court an explanation for his default. The deadline was approaching at the time of the tanker fire.

During the same period, Leimer was having trouble paying off other debts, according to court records and business associates.

He owed $17,000 to a radio station that had run his wine commercials, and he owed about $8,000 to several New England wineries.

Leimer first came to the United States from Vienna in the early 1960s, and soon after became a U.S. citizen.

He bought his upper Montgomery County home for $60,000 in 1976, land records show. The split-level home on a cul-de-sac, with a large motorboat on the front lawn and a gazebo in the back, has an extensive alarm and floodlight warning system that neighbors say Leimer installed when he bought a home computer for his wine business.

"He was a very good talker," said a friend who asked not to be identified. "If he had had time he could have delivered on his promises. But he spread himself too thin.

"He was not a malicious person. He was a good-hearted person, seriously. Basically he put himself in an awkward position with his mouth."