U.S. safety regulators have closed an investigation into steering problems in more than 500,000 Ford full-size cars without seeking a recall. The National Highway Traffic Safety Administration began investigating problems in the 2004 through 2007 Ford Crown Victoria and Mercury Grand Marquis and Marauder cars in July. The agency was looking into complaints that a heat shield could rust, dislodge and cause the steering shaft to jam. Investigators reviewed 18 complaints about the problem and found that it occurred infrequently, only in 1.6 vehicles per 100,000. The agency says one report of a rollover crash on a highway entrance ramp could not be verified. The agency also says six of the complaints came from one state police agency which fixed the problems and hasn't had any further trouble.