80's SBC. It didn't have a heater valve so I installed one. It's a 4port. The heater hose to the heater core come off the front pass intake. With the valve installed the Temp gauge fluctuates from normal 180 to almost 260. Idle or driving it doesn't matter. So i ran the heater hose from the intake to the Radiator.Temp gauge seems steady. So I pinched the heater hose off & the fluctuation came back. What would gauge this?