The truck is running great, but when the heater is on, the engine temp spikes? Its blowing nice hot heat, what what would cause that?

The temp thing is broken or came off under the dash, so I have to manually close and open the valve under the hood.

Soon as I turn the heat off, engine temp goes right back down, and thermostat opens, but with heat on, thermostat seems to stay shut? Even with the two-stage?

Its a bummer I can't have heat up here in Wisconsin.

It's hard to beat old trucks!!