This was an incident. no crash, and no injury. the iac (idle air control) valve failed during start attempts on the following vehicle: 2001 ford explorer sport, 4.0l sohc engine, mileage = 39,000. the valve failure mode was closed (no idle air) so the engine would not idle unless the primary intake throttle plate was held open by maintaining pressure on the throttle. failure of engine to idle during a moving condition may cause lack of vacuum for brake boost and unusually high brake forces, as this vehicle has a vacuum brake boost. from reviewing various on-line forums, it appears this is a common problem with the iac valve, which is used on several model/year/engine family combinations. *nm