Thanks for the info. During the overheating, I was able to hear a hissing sound from the dash just under the windshield, which I presume was steam entering the bladder. The heater core was making more of a sputtering noise, like what you hear when you sprinkle water onto a hot frying pan.
Seems you nailed it. The thermovalve, or its connection to the washer reservoir tubing, is undoubtedly the culprit.
I'm going to go ahead and replace it now rather than wait, since I have a feeling I;ll need warm washer fluid before winter ends.
I just completed my stress test of the (temporary) bypass: Drove from Fairfax, VA to Capitol Hill in Wash. DC & Back, a trip that normally causes considerable heating. The coolant temp never exceeded about 84 -- which is almost unbelievable. It always creeps close to 95/100 in Wash DC Driving.
Thatnks again for your help.
2001 E430 4matic
Gone but not Forgotten:
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black