Machining errors may cause premature bearing wear and engine failure.
Hyundai and Kia have launched recalls for nearly 1.2 million vehicles sold in the US market, addressing a defect that can lead to engine failure.
Machining errors during the Theta engine's crankshaft manufacturing process can leave metal debris in oil passages, restricting oil flow to bearings. A reduction in oil flow can allow the bearings to overheat and prematurely wear.
"A worn connecting rod bearing will produce a metallic, cyclic knocking noise from the engine which increases in frequency as the engine rpm increases," Hyundai wrote in its original recall for the problem, issued late in 2015. "If the vehicle continues to be driven with a worn connecting rod bearing, the bearing can fail, and the vehicle could stall while in motion."
Aside from the noise, drivers might also be warned of impending failure via the oil pressure lamp in the instrument cluster.
The expanded campaign affects the 2013-2014 Hyundai Sonata and Santa Fe Sport and Kia's 2011-2014 Optima, 2012-2014 Sorento and 2011-2013 Sportage. Dealers will inspect and replace the engines if necessary.