General Motors admitted yesterday that there are nine potentially major defects in its entire new line of X-body compact cars, scheduled to go on sale next week following a multimillion-dollar advertising campaign.

The world's largest automaker confirmed that it had ordered its dealers to repair the defects hurriedly. The company said no cars would be allowed to leave dealerships unless they had been fixed.

The problems range from misrouted fuel lines that can cause fires under the hood to spare tires that will not work on front wheels. Another problem is defective welding on the rear stabilizer bars.

GM rushed to make public the problems with its new X-body front-wheel-drive compacts after consumer advocate Ralph Nader, tipped to the problems by worried GM dealers, warned the company that he would make the details public.

At a news conference yesterday, Nader called on the Department of Transportation to launch a full recall investigation into 11 possible defects in the cars and asked GM to provide a written certificate to all purchasers acknowledging that the cars had defects that were corrected. This would place responsibility for the repairs on the automaker. Nader estimated the costs of the repairs at "under $125" per car.

The four new front-wheel-drive cars involved are the Chevrolet Citation, Pontiac Phoenix, Oldsmobile Omega and Buick Skylark. A GM spokesman said the company felt the written certificates were unnecessary.

GM said that an estimated 50,000 of the cars already had been shipped to the dealers and would require repairs, but not all of the cars need all of the fixes. The company added that later production models would have the flaws corrected.

Meanwhile, the National Highway Traffic Safety Administration said it had been warned of only four of the problems by GM, with three of those warnings coming only last week. Automakers are required by law to report possible safety defects to NHTSA within a week of discovery.

But NHTSA Administrator Joan Claybrook said her agency was looking into the possibility that the company deliberately tried to skirt that regulation by classifying certain defects as non-safety-related, and informing dealers of the fixes orally, and not in writing.

It is GM's contention, for example, that the fact that the spare tire cannot be effectively used on the front wheel without rubbing against the brake apparatus is not a safety-related defect.

Claybrook said her agency has "initiated an inquiry" involving allegations that the company failed to " notify us in a timely manner or if there was any intent not to notify us of safety problems."

On Thursday, the B.F. Goodrich tire company notified the NHTSA of two problems involving its special spare tires for the X-body series. Besides the rubbing problem, the tire company said the sidewalls on the spares may not be strong enough to stand up to the added pressures of front-wheel drive.

But it was the visit of GM officials to various auto dealers that opened the door to GM's problems. Auto dealers became concerned that they could be held responsible for problems concerning the fixes, especially since the company was apparently unwilling to put many of the fix orders in writing.

GM contends that the problems surfaced when they did because of an extraordinary testing procedure the company is using for the first time. GM built 150 test units of the cars on the production line that will be used for the entire line, and began testing those cars. Consequently, defects that normally would have been found after new production cars were on the road were caught early.

The four defects GM pointed out to NHTSA were a front-seat latch that doesn't work properly, a rear stabilizing bar that is inadequately welded, a misrouted fuel hose that could leak and cause a fire under the hood and a clutch cable that could interfere with the brake line.

Other components the company has ordered repaired are defective front body mounts and front tie-down holds (used in holding the cars down during transport), which could crack a floor pan under the engine, faulty carburetor parts on four-cylinder cars, the spare wheel problems and front alignment bolts.

Nader and Archie Richardson Jr., president of the Automobile Owners Action Council, said they had identified two more problems: an insulation package around the steering column that makes it difficult to move the driver's right foot from the gas pedal to the brake pedal, and seat tracks that do not meet federal safety standards.

Problems in new model cars are not unusual. Both Ford Motor Co., with the Mustang and Capri, and Chrysler Corp., with the Omni and Horizon, had to issue huge recalls of their early production cars for safety problems. But the GM x-body csae is by far the most widespread problem even encountered by an automaker at the outset of a new line. CAPTION: Picture, The new GM X-body autos of Pontiac, Oldsmobile, Buick and Chevrolet. In foreground: the Pontiac Phoenix. AP