Hi there, This has been discussed countless times so do a few searches and you'll find its due to a faulty thermostat jammed open. You can either replace it (expensive and very difficult) or do the mod of a second thermostat in line, which I've done a video howto on it. Good luck.
Check fuel filter housing where primer is. Fuel heater overheats and distorts housing letting air in. Need to replace parts and new wiring pigtail kit. Part numbers are 68043086ab wiring kit, 68043089AA housing assy. There was a tsb in the USA regarding the diesel liberty and from research problems are possible up to 07 model year. Hope this helps. To check. Remove 2 plugs on top of housing. Press primer until hard. If any fuel residue or wetness is in the socket housing then it needs to be replaced.