The government has seized so many cars, trucks, boats, helicopters and airplanes from suspected smugglers that it doesn't always have a place to put them, a General Accounting Office study has found.

As a result, the GAO said, in 1981 and 1982 the Immigration and Naturalization Service was forced to stop seizing vehicles used to smuggle aliens into southern California and Texas. The Drug Enforcement Administration also has had problems storing boats and airplanes confiscated from suspected drug traffickers.

The DEA, INS and U.S. Customs Service seized a record 9,035 vehicles during 1981. By April, 1982, it had disposed of all but 3,665 cars and trucks, 692 boats and 161 aircraft worth a combined total of $82.1 million.

Agencies use the proceeds from sales of seized property to pay for its storage and maintenance, and the sales usually cover the costs. But the government still loses several million dollars each year, the GAO said, because agencies do a poor job of maintaining and selling seized property.

"Engines freeze, batteries die, seals shrink and leak oil, boats sink, salt air and water corrode metal surfaces, barnacles accumulate on boat hulls and windows crack from heat," the report said. It added that the government doesn't try very hard to prevent the problems, or "even wash a car or inflate its tires" before selling it.

As a result, the government earns only 58 percent of the value of the cars that its sells. The value of boats and airplanes is even less--43 percent and 35 percent, respectively.

Some examples:

* A 40-foot boat valued by appraisers at $78,000 was left in the water for 15 months after U.S. Customs agents arrested its owners on drug-related charges. The boat's value had dropped $26,000 when it was sold because barnacles had damaged its hull and its engine seals had cracked.

* A 28-foot racing boat seized in Miami was choppped into pieces by a vandal after it was left in an unguarded area for several months. Originally appraised at $30,000, the boat was sold as scrap for $5,000.

* A 1961 Beechcraft airplane worth $50,000 was sold for $4,000 last year by a U.S. marshal who had advertised it one day in one small newspaper. Another marshal forgot to notify several persons who had inquired about a boat worth $150,000. The boat eventually was sold for $13,000 at auction, even though a potential buyer who wasn't told about the sale had indicated he would pay up to $40,000.

The GAO said it also found federal employes keeping confiscated vehicles for their own use. Two DEA agents each retained expensive Mercedes-Benz sedans taken from suspected drug dealers, and an Army officer acquired a 1976 Rolls Royce.

The GAO first complained about the care and disposal of confiscated vehicles in 1977. Since then several congressional committees, task forces, the President's Private Sector Survey on Cost Control and agency auditors also have issued critical reports, the GAO noted, but little has been done.

One problem is the length of time it takes an agency to forfeit vehicles. If a vehicle is worth less than $10,000, the government can sell the vehicle without going to court--usually within eight months. But to win possession of more expensive vehicles the government must go to court, and that usually takes at least 18 months.

The GAO recommended that the limit be raised to $100,000, in part because drug traffickers usually use more expensive vehicles. For example, agents recently confiscated a Learjet worth $3 million, a Boeing 707 passenger airplane and a yacht appraised at $900,000.

Cumbersome accounting procedures also are a problem, the GAO said. Agencies only can use the proceeds from a sale to pay the storage and maintenance costs they incurred in the same fiscal year. Since most vehicles are stored for more than a year, the agency can be only partially reimbursed. Because the excess funds go into the Treasury, the agencies have little incentive to try to get top dollar, the GAO said.

Since agencies must pay their maintenance and storage costs out of their current budgets, they generally would have to cut back on their enforcement efforts if they wanted to devote more funds to the sales effort.

For instance, an INS official in Laredo, Tex., said it would take two or three agents a week just to wash the 552 cars that his office has in storage.

The Violent Crime and Drug Enforcement Improvements Act of 1982 would have streamlined the forfeiture process, but President Reagan vetoed it in January because he opposed other parts of the bill. Those provisions have been shifted to the Comprehensive Crime Control Act of 1983, which is awaiting markup by the Senate Judiciary Committee.