The National Highway Traffic Safety Administration said yesterday that it might expand its investigation into "sudden acceleration" mishaps involving Audi 5000S cars with three-speed automatic transmissions.

"Sudden acceleration" occurs when a vehicle moves forward at high speed from a standstill in reaction to light or moderate pressure on the accelerator pedal.

NHTSA currently is collecting acceleration-malfunction data on 1984- and 1985-model Audi 5000S cars sold in the United States. But the agency said yesterday that it might expand that investigation to include 1978 through 1986 models of the car, because of a petition filed by New York State in conjunction with the Washington-based Center for Auto Safety.

The petition says that one out of every 500 Audi 5000S cars sold in the United States since 1978 has been involved in sudden-acceleration accidents, resulting in some serious injuries.

Officials of Volkswagen of America, which distributes Audi cars in this country, said they are "cooperating fully" with the NHTSA. But they said that, so far, they have not been able to find a technical defect in the cars.

At the same time, General Motors Corp. said it is recalling about 136,000 of its 1985-model Buick Somerset, Oldsmobile Calais and Pontiac Grand Am models equipped with 2.5-liter engines to replace the throttle return assembly spring.

GM said that if both springs in the throttle assembly were to fail, the gas pedal might not return to the idle position, causing the car to maintain its speed even if the driver removes his foot from the accelerator.

GM spokesman David Hudgens said the problem would not cause the car to accelerate unexpectedly.