where is the overfolw tank?
The W129's SL600 has the overflow tank under the front fender well. Got to remove fender liner to get to it. They run hot by design. Try cleaning the exterior of the radiator to make sure no road gunk is on it. Also be sure the antifreeze mixture is not too stout. A 40% AF/water works best all around. Less AF in the mix gives better cooling.