Hyundai/Kia is well known for using substandard metallurgy in their components. It's no surprise the low quality valves failed.
They build impressive engines that mimic modern designs perfectly, but use the lowest quality metals, coatings and components. The result is questionable longevity. Some last, some do not.
They build impressive engines that mimic modern designs perfectly, but use the lowest quality metals, coatings and components. The result is questionable longevity. Some last, some do not.