One problem related to crankcase (pcv) has been reported for the 1999 Ford Explorer. The most recently reported issues are listed below.
While driving, throttle/ air bypass valve malfunctions which causes accelerator pedal to stick, and pedal goes nearly to the floor. Also pedal has to be pumped before it releases. The dealer said they can fix the problem, but the vehicle still hesitates and there is rough idle, the lower ball joints failed, and were replaced.