I have a 4.6 litre V-8 in a 1997 f150. The engine heats up to the red area and boils over when I first drive my truck after it sits for awhile. Today i drove it to work and about 3 miles from home it heats up and the heater gets cold and then it seems like the thermostat opens and the truck temperature is fine for the rest of the drive to work. The drive is another 30 miles over a mountain pass. The same thing happened on the drive home. The truck has been doing this for about a month. I have change the thermostat, rad cap and flushed the engine. Any suggestions? Maybe faulty new thermostat.

 

Sounds like the new thermostat is crap. I'd check it out.

 

you put the thermostat in the right way?? sounds stupid but ive made that mistake and then thought i had a major problem.

 

Yeah good place to start. Check it in boiling water. Did you put a Stant in it? Is your upper radiator hose pumping firm(cold) before the thermo opens?