06-12-15, 11:03 PM
I'd guess your thermostat isn't opening. The coolant gets pumped around the engine until the thermostat warms up and opens and allows it to flow around the rad to cool it down. If the thermostat is stuck closed the rad will stay cold and the engine will overheat. Did your original engine overheat by any chance? Or the coolant is really low but I'm sure you've checked that already.