WASHINGTON (AP) _ Federal safety officials said Tuesday they have begun investigating allegations that a defective computer on 1987 and 1988-model Plymouth Sundance and Dodge Shadow automobiles causes the throttle to stick.

The National Highway Traffic Safety Administration said it has received 89 unconfirmed complaints about the problem, including reports of four accidents, one of which caused an injury.

The throttle is controlled in part by a small computer system in the car. The sticking reportedly makes the vehicle slow to return to idle and causes a reduction in engine braking - the slowing of a vehicle by the drag of the engine, NHTSA said.

Meantime, the agency said it has ended a five-year investigation into alleged power-brake failure in GM's 1980 X-body cars, finding that ''no defect trend'' existed.

The allegations of a defect had been cited as contributing to a brake problem linked to 1,417 accidents, 18 deaths and 400 injuries, NHTSA said.

A federal court after a lengthy trial refused to order a recall of 1.1 million 1980 X-body cars.

NHTSA opened its inquiry into the Sundance and Shadow vehicles, a total of 345,000 cars, as an engineering analysis, the second highest level of investigation.

Chrysler spokeswoman Karen Stewart said the company was cooperating with the investigation. The manufacturer, in a service bulletin on the problem earlier this year, described the problem as a failure of the engine to return to a lower rate of idle if it were driven while cold.

''In terms of ever feeling that you're out of control of the car and you can't stop it - No. ... It is just slightly higher than the idle normally would be when the engine is warm,'' Stewart said.

She said the company believes no braking problem is involved.

NHTSA also began seven lower-level investigations into alleged defects involving cars made by General Motors Corp., Ford Motor Co. and two foreign manufacturers, Subaru and Saab.

One involved three complaints of the wheel and hub assembly falling off 1986-1987 Pontiac Fieros because of metal fatigue.

The problem resulted in three accidents but no injuries, according to unconfirmed reports received by NHTSA. The preliminary evaluation involves 104,000 vehicles.

GM, the maker of the cars, pledged to cooperate with the NHTSA investigation, but spokesman John Anderson added, ''Just because there is an investigation doesn't necessarily mean there is a defect.''

NHTSA also opened preliminary evaluations into allegations that:

-The power-steering pump on 1987 GM Firebirds and Camaros fractures, causing a loss of power steering. The investigation, based on 10 complaints, involves 250,000 vehicles.

-The bracket bolts holding the steering column on the 1988 Ford F-150 light truck come off, causing the column to fall. The investigation, based on two complaints, involves 80,000 vehicles.

-The adjustable front-seat headrest on the 1986-1988 Ford Taurus and Sable cannot be kept in a raised position. The investigation, based on three complaints, involves about 1.2 million vehicles.

-The front-seat bracket of the 1986-1987 Taurus and Sable fails, allowing the seat to move nd making control of the cars difficult. The investigation, based on four complaints, involves 420,000 vehicles.

-A faulty oil-pressure gauge sensor on the 1984 Subaru GL fails to work, allowing oil to leak in the engine compartment, where it causes a fire. The investigation, based on three complaints, involves 157,000 vehicles.

-A faulty cable can cause the battery to short-circuit on the 1988 Saab 900 Turbo. The investigation in the 5,000 cars was prompted by a Saab service bulletin.

Besides the GM X-cars, NHTSA also closed without action investigations into alleged:

-Cruise-control response in GM's 1988 G-series vans with 6.2-liter diesel engines.

-Driver-seat failure in Ford's 1988 Lincoln Mark VII.

-Rear-wheel lockup in Ford's 1983-1984 F-250 trucks.

-Windshield wiper failure on Chrysler's 1987 Jeep Wrangler.

-Seat failure in Mitsubishi's 1986 Montero.