TOYOTA CITY, Japan — Toyota Motor Corp. has assigned 1,000 engineers to help analyze quality problems and is extending time devoted to testing new models by an average of four weeks in an effort to head off glitches before vehicles enter production, a senior Toyota engineering executive told The Wall Street Journal.

Changes in overall development time could vary depending on the size of the car-development program, Executive Vice President Takeshi Uchiyamada, Toyota's chief engineer globally, told reporters Wednesday at the auto maker's headquarters here.

The moves follow an embarrassing quality crisis earlier this year that resulted in Toyota briefly halting sales of certain vehicles in the U.S. and led to global recalls of more than 8.5 million vehicles, mostly for sudden-acceleratoin complaints. The company also was fined by U.S. safety regulators, who alleged Toyota didn't act fact enough to report and correct defects.

"In the aftermath of the recent issues we have learned many things and some of them have forced us to change our operations," Mr. Uchiyamada said

Toyota has added a new layer of assistant managers to its structure to spread the vehicle-development burden, Mr. Uchiyamada said.

Toyota led journalists through its testing labs to show its efforts to track down possible causes of unintended acceleration. Company engineering and quality executives say there is no evidence that flaws in its electronic throttle-control system could cause the problem.

Toyota showed electromagnetic interference tests as well as hardware and software tests run on its throttle systems, and said that none of its tests have shown any flaws. Toyota has eight electromagnetic testing chambers, far more than other auto makers, officials said.

Mr. Uchimayada said he would like to reduce the amount of outside engineers working on research and development, but the process will take time. The company must wait for contracts to expire before it can bring the work inside the company. A company executive said the target is 10% outside engineering contractors, down from 30% now.

The changes in development come as Toyota said in late June that it will recall up to 270,000 of its top-end Lexus models globally for an engine problem that could cause stalling. In recent months it has recalled tens of thousands of other vehicles for steering, braking and gas tank flaws.

"I think the big question is, is it over? Will there be more revelations?" said Jeremy Anwyl, chief executive officer of car research service Edmunds.com in an interview last week. Toyota's biggest issue was failing to report the problems in a timely manner, he said.

Still, Toyota is battling perceptions that its fixes, including fasteners for floor mats and new accelerator pedals, don't actually address the problems.

Toyota said it has investigated 3,600 vehicles in the U.S. for customer complaints and found nothing related to electronics, executives said. The U.S. National Highway Traffic Safety Administration also has found no evidence of electronic flaws in its own investigation so far.

Toyota spent roughly $4 billion to repair vehicles involved in recalls in the fiscal year that ended March 31. CEO Akio Toyoda apologized to shareholders last month for the flaws in its vehicles that led to the recall.