It might be flowing TOO FAST and TOO EARLY, and not re-closing/restricting flow enough for the radiator to have time to transfer heat to the air properly. Sometimes a standard flow t-stat, in maybe a higher (180?) degree +++++++++ What mixture of coolant did you install? You want 50/50. Some think stronger is better. This is completely wrong. Anything stronger than 50/50 that has MUCH lower cooling capacity than a stronger mix because ironcally Anti-freeze is TERRIBLE at cooling. WATER is the absolute necessary ingredient in the mix for proper heat transfer.
comments on abv ...yes , many times it flows TOO fast...as when people remove T stat for 'better flow'...water has not enough time in radiator to cool down.
50/50 is for modern AC cars...if your car does not have AC , best cooling of coolant and water is 33 % coolant 67% water...the most efficient cooling mixture...but AC cars use a higher % for lubrication IIRC.
and other ideas...is your fan a puller or pusher? pusher fans are not good ,they also block radiator.
was this your car before the rebuild...i.e. did you drive it before w/o over heating issues? Do you have the fillers? the lower filler, the masticated on ea side of bottom of radiator? and the X filler plates?
