By TOM KRISHER
AP Auto Writer
DETROIT (AP) — U.S. auto safety regulators have stepped up a series of investigations into multiple engine fires that have plagued Hyundai and Kia vehicles for more than six years. The National Highway Traffic Safety Administration says a new engineering analysis investigation covers more than 3 million vehicles from the 2011 through 2016 model years. The agency has 161 complaints of engine fires, some of which occurred in vehicles that already have been recalled. Engine failures and fires have dogged the Korean automakers’ vehicles September of 2015 when it issued an engine failure recall. Since then it has issued at least eight more recalls for a host of engine problems, according to NHTSA documents posted on its website Monday.