Wanted to add to symptoms for v6 SOHC engine. It also when the
coolant level is full. Such as after putting a new thermostat in.

It will push the coolant into the overflow tank. Won't draw it back out.

And if I leave the radiator cap open it will still make the gauge move
towards overheating. But will not go all the way. Also when it's going
to overheat the heater will start to blow cool air. It will be cool for about
two miles. Then after pull over and rev it up a couple of times heater air
turns warm gauge drops and I could drive 500 miles without it doing it
again. Had it to at least 4 mechanics. Heads check out good?????