There is a technical service bulletin for the V8 engines regarding a possible problem with the spark plug being ejected from the cylinder head, damaging the threads in the spark plug hole. If the vehicle is under warranty, cylinder head replacement is recommended. If the vehicle is out of warranty, Ford authorized the use of a specific thread insert to repair the head.
Problems for specific Ford Expedition years:
Car problem reports
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Most reported 1999 Ford Expedition problems
The PCV hose may crack, causing the vehicle to idle roughly, this may or may not illuminate the Check Engine Light.
Intermittent rough idle may be caused by the EGR sensor sticking, causing the EGR valve to stay slightly open. Our technicians tell us the valve and sensor must be replaced together to correct this concern.
humming noise from engine when starting cold. faulty idle air control valve causing noise,can tap on valve and noise will go away. must replace valve to fix.