If this happened after the cooling system was serviced, it could have an air pocket in the cooling system. Due to design, the thermostat sits high in the system, and air gets trapped around it. It may not be opening, because of this. It can either be pushed open with the upper hose off, and filled through there, or even the heater hose that comes off the housing and makes a left at the rear of the engine. With some liquid around the thermostat, it will transfer heat to open it. Head gasket leaks are not as common on a 4.9l, but it is possible. If you are certain there is no air in the system, have it pressure tested and block tested.
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Wednesday, December 2nd, 2009 AT 10:08 PM