LOS ANGELES - Toyota Motor Corp. said it hasn't found any evidence its electronic throttle control caused unintended acceleration after engineers and technicians investigated more than 4,000 U.S. vehicles whose drivers made such complaints, Bloomberg reported.

“Toyota has not found a single case in which electronics would lead to sudden unintended acceleration,” Steve St. Angelo, Toyota's North American chief quality officer, said in a conference call. Toyota has reviewed 4,200 individual acceleration-related complaints so far, he said.

The world's largest automaker now is working to improve its brand image and quality after global recalls of more than 8 million vehicles for defects linked to unintended acceleration.

Some media reports and safety advocates have suggested Toyota's electronic throttle-control system plays a role in sudden acceleration -- an assertion Toyota has denied from the beginning of the crisis.

Customer complaints of sudden-acceleration incidents have dropped 80 percent since April, Toyota said in a statement.

The automaker said it has added so-called brake-override control to 84 percent of Toyota, Lexus and Scion vehicles now on sale in the U.S. and will be the first manufacturer to offer the safety technology across its lineup. The software is intended to ensure that should the accelerator become stuck, it will be shut off when the brake pedal is pressed.

About 5 million recall-related repairs to fix sticky gas pedals and replace floor mats at risk of jamming the accelerator have been completed so far, Toyota said today.

The company is working more closely with suppliers in North America to review parts designs and manufacturing processes and to test components more thoroughly, St. Angelo said.

Toyota is cooperating with federal probes that continue to look into potential electronics flaws and how the company handled past recalls, and doesn't know when they'll conclude, St. Angelo said.