WASHINGTON (AP) _ The National Highway Traffic Safety Administration has begun an engineering analysis of throttle-control problems in 800,000 1986-87 General Motors Corp. cars, according to a letter released Thursday.

The agency had been conducting a preliminary evaluation. An engineering analysis is the next step in a process that can lead to a recall.

NHTSA, in a letter released by the Center for Auto Safety, a Washington- based consumers group, said the investigation ''includes, among other phenomena, that of so-called sudden acceleration.''

The investigation concerns H-body cars - the Buick LeSabre, Oldsmobile Delta 88 and Pontiac Bonneville.

The center said it knows of 62 accidents resulting in 48 injuries and one death when H-cars suddenly accelerated after the gearshift lever was moved out of the park position.

The engineering analysis is the sixth formal investigation being conducted by NHTSA into sudden acceleration or engine surge in cars, the center said. Some 4.5 million vehicles are being investigated for such reports.

NHTSA's preliminary investigation of the H-cars had been labeled ''throttle system failure.'' It was changed to ''throttle control'' for the engineering analysis.

General Motors, through spokesman Bill Noack, said it would cooperate fully with NHTSA's investigation of the reports.

''We have been studying those for a good many years in an attempt to find a root cause, and we haven't been able to find such a root cause,'' Noack said.

NHTSA also upgraded to an engineering analysis its investigation of the seat attachments to 120,000 1985-86 Chrysler Corp. H-, G-, K- and E-cars, including the Dodge Lancer and Chrysler LeBaron.

Drivers have reported the power seats in the cars fall backward.