I have a new 383 small block with less than 100 miles on it. I am using a 180 degree high flow thermostat. My problem is when I start the car from being cold the engine temp goes up to 220 every time and them down to 180 and will stay at 180 the rest of the time. This is the 3rd thermostat and they have all done the same thing. I know its running that hot because it loses antifreeze out the overflow tube. I am using a new mechanical autometer temp gauge on it. The radiator is a new triple core. I tried a regular 180 thermostat and it has a hard time keeping up and runs 190 to 210. The sending unit is next to the number 1 cylinder on the head. Any ideas ?:confused:
Thanks

Have you tryed moving the temp sensor from the head to the intake manifold coolant crossover beside the stat hsg?This would be a more balanced overall coolant temp reading.

Have you tryed moving the temp sensor from the head to the intake manifold coolant crossover beside the stat hsg?This would be a more balanced overall coolant temp reading.
X2.... Plus is there air in the system.

My problem is when I start the car from being cold the engine temp goes up to 220 every time and them down to 180 and will stay at 180 the rest of the time
this could because the head is before the thermostat, and heats fast, and then as the flow goes thru, cools ...this would be espec noticable if fire up and let idle, and there is not a lot of water going thru at idle...if u fired up and drove around or held rpms up higher I doubt if u would hardly notice it......220 is not excessive

I know its running that hot because it loses antifreeze out the overflow tube.
If u top up each time , then the radiator is over full, and yes it will over flow, and not because it is running to hot...the correct level is about 2" below then neck.
It should over flow as the water gets hot....then again when u shut down after a good run...the water remaining static in the block, get very hot, and expands further.