The US government's auto safety regulator has ended a 2 1/2-year investigation into Ford engine failures after the company replaced engines or extended the warranty on some vehicles. The National Highway Traffic Safety Administration says in documents posted Monday on its website that its analysis traced the problem to intake valves that can fracture inside some 2.7-liter and 3-liter turbocharged engines. Documents say the probe opened in May of 2022 ended up covering more than 411,000 vehicles from the 2021 and 2022 model years including the Ford F-150 Bronco, Edge and Explorer as well as the Lincoln Aviator and Nautilus. The agency was looking into catastrophic engine failures caused by intake valves fracturing, dropping into the cylinder and hitting the piston. The documents say a forensic analysis of fractured valves found that when the they were made by a parts supplier, the temperature got too high, making them brittle and likely to fracture during normal engine use. An anal