I am helping a friend with her '74 450SL. One of her problems is that the engine is running real hot. I just drained the radiator and the block, as her old coolant was disgusting looking and the coolant to water ratio was off. So I refilled the system, ensuring a 50/50 mix.
Anyways, here is my question. I added all that she would take and let the car idle a good long while. The temp gauge gradually crept up to well above 210 Degs. The upper radiator hose got very hot, but the lower hose was cold. I mean, not even luke warm. This was even after an indicated 200+ degs. The engine seems to have a reverse flow cooling system. The lower radiator hose goes to/comes from the thermostat.

I plan on taking the T-stat out and do the boiling water test. In the mean time, what are everyone's thoughts on this. From what I read in the service manual, the t-stat should be opening at 160 degs. 160 degs, I certainly would have felt when I grabbed the lower rad hose.

I drove the car last night, on the highway and the eng. temp stayed in the 200-215 range. Though it was at night and much cooler. My friend said that on some of these 98 deg days we have had, the temp got up to just below the red line. My thought is that the t-stat is partially closed.

Any thoughts and tips would be greatly appreciated.

__________________
1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member