[chevyIIpost]

Another element to consider is air in system. Air will become steam and steam doesn't cool anything. I normally run the engine at 1500 rpm and get the t-stat to open and purge the air out through the radiator fill neck. I have seen the top of the radiator ending up lower than the t-stat housing making it difficult to get all the air out. You will need to run an over flow bottle that lets the system pull coolant from the bottom to keep air out of the system. While running the engine at 1500 rpm and filling it note the water flow. With the stat open it should be a strong flow of water moving in the radiator tank at the fill neck. Also don't forget the heater core can trap air. Make certain the heater is on and the water valve is open to purge the air out of it. Check that the cooling fan is off at highway speeds. I have personally seen an electric cooling fan restrict air flow at road speed and cause the engine to run hot. It is a system and a parts must do there job together. You just have to find the part of the system that is not co-operating. Water flow, Air flow, and system controls(t-stat & fan stat).