Pcmaher,
Nothing to pardon; a bad visco fan clutch DOES cause overheating; but it usually does so UNDER 30 mph. I'm overheating more at higher speeds.

Tomguy,
What you're saying is what I had always thought prior to writing my "SOS" posts and looking things up on the internet. Since I was a teen, I'd always narrowed overheating issues to 4 things: coolant, radiator, thermostat, water pump.

As I've been reading, I've heard all kinds of alternative explanations. I have written to 3 Benz posts (as I was seeking a swift response) and you very well may be the first person to say, "I think it's your water pump."

The radiator core is 7 mos old; the water pump and thermostat are less than three years and 4000 miles old. The coolant level was fine, as judging by the clear overflow tank. I note that the old thermo was a Wahler 75C (I purchased a Behr unit); I have no reason to doubt that the current water pump is a quality OEM replacement unit. I purchased a lower hose, because I ALWAYS replace related inexpensive routine items that are under pressure/load when I do a bigger job - especially if that item could possibly be causing the problem (I did not see the hose kink when I looked at it).

As I don't have a gauge to measure V belt tension here, I was loathe to remove the alternator; and that made getting the thermostat housing off a major pain (shoot, this job is a pain anyway; it has got to be the most maintenance-unfriendly system I've ever seen, as I never had to drain a rad and remove alternators to take off a thermostat on any other car I've ever owned). But my plan now is to replace the thermostat and lower hose, add water (just to test the system), flush the system (it it seems to be circulating), and then replace w/ a 50/50 mix. If I still have overheating issues, I will pull off every 30-35 miles during my 100 mi drive home, and eventually be able to replace the pump.

On the subject of water pumps, which unit? I saw a Graf and an Airtex for about the same $$$, any opinions on which is better? Is there a "THE" pump?

When I replaced the rad core, I had no qualms; it was the original radiator. Rads go bad; no worries. But now, I'm looking at parts that SHOULD NOT BE worn out or bad yet. THAT DOES worry me. Why would I have to replace these components so soon after last time?

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