Vortec heads and blocks do not have the internal coolant bypass, older heads and blocks do. That is why the 'Vortec' pumps have two 'bypass' hookups, one is for the heater core return, the other hooks to the front of the intake manifold usually under the thermostat area. That allows coolant to flow when the thermostat is closed, ie bypass. If that main bypass under the thermostat isn't hooked up on our trucks you will get steam pockets in the heads. Guess what temp the steam is at?? 260++ ... Our trucks have that cluster **** heater control hose setup that doesn't allow ANY flow thru if there is no vacuum applied to it, so there is NO coolant flow thru the heater core unless you put the heat on inside the truck. Try that as a test ... if you have your heater core hooked up that should allow the truck to stay cool enough until your thermostat opens. Older vehicles, like my Camaro, have constant coolant flow thru the heater core, acting as a bypass, so Vortec heads on older blocks with proper heater cores hooked up are fine. Lots of guys run Vortec heads, older blocks, no heater core with a water restrictor plate, that setup is fine too, constant flow. When you take the thermostat out of your truck there is constant flow, not allowing the steam pockets to form, like the water restrictor setups in older blocks or race setup engines. When your thermostat is in there, there is NO flow when it's closed and I'm betting you don't have the coolant lines hooked up right. Basically allowing no coolant to bypass the thermostat and allowing steam pockets to form. Follow??
I bet if you pull that thermostat and drill a few small holes in the perimeter of the housing it may allow enough flow, possibly enough to "fix" your issue ... I did that with my Vortec swap at one point ... some guys have luck with it, lots of guys with later model stuff like us, do not and need to properly address the bypass stuff ... which is what I had to do.
