I have been trying to figure out a heater issue in my 2003 f150 v8 4.6L. The heater will initially blow warm arm when the truck first starts for 5 min then start blowing cold air. Then the temp gauge will raise rapidly to hot and peak out. then return to nl at 1/2 position. The truck never over heats. the heater then starts blowing hot air and is fine afterwards. It may take 10min of driving for this to occur. Both heater hoses are hot during this entire time. I have flushed the coolant system changed to thermostat. I removed the thermostat and drove the car around and the temp remained barely over the cold mark but the heater did blow warm air the whole time. Temp gauge never went to the hot peak mark. I verified that the new thermostat opens in a pan of boiling water. :evil: What gives?

 

You probably have an air lock in the cooling system. It needs to be "burped". Take the cap off, start it up, and let it idle for a while, top off as needed.