The U.S. government is investigating whether Hyundai and Kia moved quickly enough to recall more than 1.6 million cars and SUVs because the engines can stall, increasing the risk of a crash.
The National Highway Traffic Safety Administration said it's looking into three recalls by the Korean brands. The agency also said it's investigating whether the automakers followed safety reporting requirements.
Hyundai recalled about 470,000 vehicles in September of 2015 because debris from manufacturing can restrict oil flow to connecting rod bearings. That can cause bearings in the four-cylinder engines to wear and fail. The repair is an expensive engine block replacement.
Last March Hyundai and Kia issued two more recalls covering 1.2 million additional vehicles with the same engine problem.
The companies said they're cooperating with the investigation.
"We act quickly and efficiently to address and fix problems for impacted customers, while following all government regulations and requirements," Hyundai said in a statement Monday.
If the agency finds that the companies, both owned by Hyundai, moved too slowly to vehicles, then it can issue fines or order additional recalls.
NHTSA, in documents posted on its website over the weekend, said Hyundai limited vehicles in the 2015 recall to a group manufactured prior to April 12, 2012. The company said it solved the manufacturing problem after that date.
Kia said at the time of the 2015 recall that its 2.4-liter and 2-liter "Theta II" engines that were the same design as Hyundai's were not recalled then because they were made on a different production line and didn't have the same issue.
Then both companies issued further recalls 18 months later for the same problem, including models the automakers originally said weren't affected by the problem, agency investigators wrote in the documents.
The documents don't list any crashes or injuries from the problem.
The recalls from March cover some of the Korean automakers' most popular models in the U.S. and Canada, including 2013 and 2014 Hyundai Santa Fe Sport SUVs and Sonata midsize cars. Also covered are Kia Optima midsize cars from 2011 through 2014, Kia Sportage SUVs from 2011 through 2013 and Kia Sorento SUVs from 2012 through 2014.
There was a similar recall in South Korea of the Hyundai Grandeur and Sonata sedans and Kia's K5, K7 and Sportage with engines produced from 2009 to 2013.
Owners will hear a knocking sound from the engine that increases in frequency as the engine speed rises. They also could get engine warning lights on their dashboard, Hyundai said. Those with problems should contact their dealers.
The recall of the U.S. engines made at Hyundai's engine plant in Alabama, was to start this month.
If their vehicles have a problem, owners will hear a knocking sound from the engine that increases in frequency as the engine speed rises, Hyundai said. They also could get engine warning lights on their dashboard. Those with problems should contact their dealers.
The recall was to start on May 19.
Kia is a smaller affiliate of Hyundai. Together they are the world's fifth-largest automaker.