I have 260 V8 that I put an edelbrock intake on. Ever since installing it, the radiator wants to overflow, even when it's cold. It doesn't seem to do it until it's under a load. If I let it run for 20-30 mins until it's warm it doesn't overflow but I can get in the car and drive it and it back flows antifreeze into the overflow, cold antifreeze usually. I have replaced the thermostat, gone through nearly 6 gallons of antifreeze which make me believe the air should be purged from the radiator/ system. I have insured that no hoses are pinched and I am fairly confident that the gasket didn't slip or move when installing the intake manifold. What are the possible problems? Anyone else had these type issues? Thanks in advance for your time.

 

If you drive for a half hour and still have cool antifreeze to the touch? Sounds like a t-stat issue.... No flow.
Is the t-stat installed backwards?... It happens.
I'd remove the t-stat... Drop it in a pot of boiling water and verify its not defective before buying a new one.
If everything checks out and reinstalled with the rad cap off fill the rad to the top start er up and let it circulate getting any and all of the air out off the system.
If you continue to have the problem, bypass the heater core to eliminate that factor.
I assume your water jackets were not plugged.
Other than that buddy all that's left is the impeller in the water pump.
Let the elimination process begin... Good luck

 

when filling the rad if possible have the rear wheels lower than the front it helps keep some of the air bubbles out of the system when refilling. With the cap off after refill until the antifreeze starts to get warm to the touch. Then you know for sure things are circulating.