I started my 04 ford 6.0 this morning to warm it up. it ran for about 20 mins but when i got in it was blowing cold air. The air didn't get warm untill i pressed the go pedel. When i let off the pedel and the truck is just idling it goes cold again. Does anyone know whats going on?

 

is it full of coolant?

 

Yep it is full of coolant

 

thermostat may be stuck open, is the temp guage coming up?

 

A couple of issue possible. First weak water pump. When rpm's go up it pushes more coolant. Second blown head gasket pressureizing cooling system. Same thing at idle the air pockets find the heater core, as the rpm's come up the water pump can push the coolant into the heater core and the air pockets out. Third possible heater core starting to clogg.