WASHINGTON — Recalls of millions of popular Toyota cars and trucks still may "not totally" solve frightening problems of sudden, unintended acceleration, the company's American sales chief conceded yesterday, a day before the Japanese president of the world's largest automaker must confront angry U.S. lawmakers.

House members listened in silence yesterday to the tearful testimony of a woman whose car unaccountably surged to 100 mph, then they pressed James Lentz, president of Toyota Motor Sales USA Inc., on the company's efforts to find and fix the acceleration problems — actions many suggested were too late and too limited.

Lentz apologized repeatedly for safety defects that led to recalls of about 8.5 million Toyota cars and trucks, and he acknowledged the changes the company is making probably aren't the end of the story.

Putting remaining doubts to rest is of vital importance to millions more Toyota owners in the United States and elsewhere, who have continued to drive but with serious concerns about their cars. Toyota sales have suffered, too, and a small army of dealers showed up on Capitol Hill yesterday, arguing this week's high-profile hearings unfairly target their company.

"We are vigilant, and we continue to look for potential causes," Lentz told the House Energy and Commerce Committee.

That search had better continue, a number of lawmakers said, openly questioning Toyota's insistence the problems are mechanical and not linked to the vehicles' sophisticated electronics.

Without a more vigorous investigation of the possibility that electronics are involved, Texas Republican Rep. Joe Barton said of Toyota's probe: "In my opinion, it's a sham."

The U.S. government is pursuing the electronics question, Transportation Secretary Ray LaHood told the panel. "We're going to go into the weeds on that" and come up with answers, LaHood said. He said the company's recalls were important but "we don't maintain that they answer every question."

Lentz's appearance set the stage for Toyota's president — Akio Toyoda, grandson of the company's founder — to apologize in person today.

Toyoda accepts "full responsibility" for the halting steps that led to the recall, according to prepared testimony. He was expected to offer his condolences over the deaths of four San Diego family members in a crash of their Toyota in late August.

"I will do everything in my power to ensure that such a tragedy never happens again," Toyoda is expected to tell the House Government Oversight Committee. "My name is on every car. You have my personal commitment that Toyota will work vigorously and unceasingly to restore the trust of our customers.

"Quite frankly, I fear the pace at which we have grown may have been too quick" and led to safety defects at the heart of the recall, Toyoda says in his prepared testimony.

There were repeated displays of emotion at yesterday's daylong hearing — both from a Tennessee woman who survived a 2006 sudden acceleration incident when she was unable to control her runaway Lexus and from Lentz himself, who choked up while discussing the death of his own brother more than 20 years ago in a car accident.

"I know what those families go through," Lentz said.

Rhonda Smith, of Sevierville, Tenn., said her Lexus raced out of control to speeds up to 100 miles an hour, and that nothing she did to try to stop it worked — including braking and shifting into neutral. "I prayed to God to help me," she said, fighting back tears.

She said she was finally able to pull off the road onto a median and turn off the engine. She said it took a long time for Toyota to respond to her complaints and even then it was dismissive.

"Listening to Mrs. Smith, I'm embarrassed for what happened," Lentz said. Pressed by committee members as to why Toyota had not had its technicians pore over the Smith car to determine what actually caused the malfunction, Lentz said he wasn't sure where the car was now, but "we're going to go down and talk to them and get the car so that they feel satisfied. I want her and her husband to feel safe about driving our products."

LaHood told the panel the U.S. government knew the exact whereabouts of the car and would share the information with Toyota.

"All of this has been a big wake-up call for Toyota," LaHood said.

Toyota has recalled about 8.5 million vehicles worldwide — more than 6 million in the United States — since last fall because of unintended acceleration problems in multiple models and braking issues in the Prius hybrid. It is also investigating steering concerns in Corollas.