Over the last week i've been having some issues with my 2003 f150 5.4. I'll be driving along and just loose all the heat from the vents, just ice cold air, and then maybe a minute after that the engine temp gauge starts to climb. If i pull over and shut the truck off and restart, the gauge drops back down to normal and everything will be fine for a while until it all starts over again. The problem sometimes goes away for 50-60 miles but then comes back. I replaced the water pump and thermostat but neither helped. I've found that when i loose heat, one heater hose is hot and the other is cold. I also found today that if i floor it when i loose heat, it sometimes seems to take care of the problem and i get heat back and it won't overheat. i'm not loosing any coolant unless i let it get to the point where it overheats and boils over. Anyone have any ideas?

 

maybe some sort of clog somewhere

 

flush the whole cooling system, and put a new thermostat in it. make sure u dont get a gates thermostat, cuz they are s hit trust me. get an oem replacement. see if that works?