Toyota will recall approximately 778,000 vehicles that are at risk for problems with their rear suspension. But the recall hasn't started yet, because the company hasn't figured out how to fix the problem.

"The remedy is being finalized," wrote Vinnie Venugopal, Toyota's general manager of Engineering and Manufacturing in North America, in a recall acknowledgement letter sent to the National Highway Traffic Safety Administration. "Toyota will provide NHTSA with additional details at a later time."

Notifying NHTSA of the problem without a solution is an unusual step, perhaps especially so, considering Toyota says it has been monitoring the problem since 2008. The company said it would notify owners of affected RAV4 and Lexus HS 250h models by mail when they can schedule repairs.

The problem centers on improper tightening of lock nuts in the rear-vehicle tie rods. Rust has developed on the tie rods, leading to corrosion, and possibly to separation of the arm from the vehicle. Unchecked, the problem could result in loss of vehicle control, according to the defect report.

While the company says it is safe for motorists to drive the vehicles until a permanent solution is found, they should ensure the lock nuts are properly tightened.

The problem could be responsible for at least one highway accident, in which a driver reported a loud noise that immediately precipitated a loss of control. An internal Toyota investigation could not discern whether the tie-road corrosion had caused the accident.

RAV4 vehicles built in the 2006 through 2010 model years and account for 760,000 of the vehicles in the recall. Lexus 250h models number approximately 18,000 and are from the 2010 model year. Toyota did not respond to requests for comment Monday.

"Once the remedy is available, we will send a notification letter by first class mail advising owners to make an appointment with an authorized Toyota or Lexus dealer to have the remedy performed at no charge," a company spokesperson wrote in a written statement.

In its recall acknowledgement, Toyota said it had initially investigated the tie-rod corrosion problem in 2008, and decided to only monitor the problem at the time. It investigated again in August 2011 when it received additional field reports of problems. In May, it received the report of the accident.