The National Highway Traffic Safety Administration will investigate problems with stalling or surging engines in nearly 725,000 Ford Escape and Mercury Mariner SUVs and Ford Fusion and Mercury Milan sedans from the 2009 through 2011 model years.
One such problem has been described as "limp home mode," in which the affected cars or SUVs suddenly drop to 20 mph and about 900 rpms.
The mode is supposed to be triggered only by a serious engine problem and is meant to allow the driver time to maneuver to a safe place before stopping.
The agency and the automaker have received almost 1,500 complaints about the problem. There were three crashes and one injury.
The NHTSA added that the cars and SUVs may also experience power surges when the engine revolutions fluctuate to prevent stalling during limp mode, the agency said.