Navistar soared after MHR Fund Management disclosed a 13.6 percent stake in the truckmaker, more than billionaire investor Carl Icahn’s 11.9 percent holding. MHR is run by Mark Rachesky, a former protege of Icahn’s. MHR “may seek to engage in discussions with management” about how the company is run, according to a regulatory filing. Icahn raised his ownership last week.