I've got a 77 450SEL w/ 127,000 mi.

I started getting hot during a 100 mi freeway drive. By the time I noticed the issue and was able to find a place to get off the freeway, I was too close to redline for comfort [So far, I haven't noticed any telltale signs of a cracked head gasket, i.e. white smoke/steam from exhaust, gunk on oil cap, or oil in coolant, thank God].

Basically, the car gets hot at 65-70 mph freeway speed (and probably at slower speed as well) about half an hour into a drive. And it gradually just gets hotter and hotter. I tried turning on the heater, but - while it possibly seems to help if it is turned on at the start of a drive - it does NOT seem to lower the temp once the car starts to overheat. That might indicate a bad water pump. My understanding is that a fan clutch should be suspected only if it is overheating at under 30 mph.

I pulled off the thermostat and did the boiling water/thermometer trick, and it begins opening at the 167 point as it should (it opens gradually, rather than "popping open," BTW).

When I first bought the car (7 mos and 3000 miles ago), I was gradually overheating, and bought a new radiator core. I note from the PO's records that he had the water pump and thermstat replaced w/ quality OEM components less than 4 years and 4000 miles ago. [In addition to the aforementioned replacement, the pump/thermostat were also replaced 9 years ago at 115000 miles (It has 127000 now)]. It is hard to believe that any of these components should be failing again so soon. Could another component be the culprit? If the water pump IS bad again, why is it failing so quickly?

BTW, I noticed a light-brownish, powdery residue at the top rad hose juncture w/ the water pump. It is in ball-shaped clumps that turn to powder at the touch. A parts store guy said this comes from using regular instead of distilled water. My fear was that someone put some kind of "stop leak" crap in the system that could have seized the pump, but the PO/original owner was a medical doctor who took the car to quality repair facilities and didn't monkey w/ it himself.

There's also some kind of temp sensor on the top of my water pump that has a "greenish" hue to it. That couldn't cause me to overheat at freeway speed, could it?

Any troubleshooting explanations would be greatly appreciated.

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Love driving my '77 450 SEL!
124,000 miles