420A 420A overheating at high RPM, full cooling system is new

[Thatguy256]

Iam going to start with what has been done since i can assume the title explains the reason for the post. I just finished a full rebuild on a 420a, new rings, bearings, head gasket, head checked then port and polished, added oversize valves. I have been unable to break in the motor due to an overheating issue. Now for the overheating troubleshooting, water pump is new, radiator is new, new radiator cap, new temp sensors, fans do work properly, new T-Stat, full system flush, compression test is good across all 4 (140 - 150). Replaced the ECU due to bad caps. I have used an infrared thermometer to verify radiator has no clogs, and temp readings are correct. T-Stat opens properly. The car will sit and idle at 180 temp all day long but the moment it starts down the highway above 2k rpm the temp slowly starts rising, come to a stop let sit running for little while and temp drops back down. Iam almost at the point of bringing the car to the scales and cut my loses.

 

[98420a]

Did you ensure that you bled all the air out of the system. Including the heater core. Also check that the cap is holding pressure. The pressure is what keeps the boiling point so high..