[TR] Overheating issue

[Rye Livingston]

This past weekend my son and I participated in the 3-day Motherload 400 Rally all around the lower part of the Sierra Mountains, next to and into Yosemite Valley, driving about 150 miles per day.  As you may have heard on the news, we're in the middle of a heat wave in California.  It was 93 Friday, 97 Saturday, and 105 Sunday.  The week before I found a small leak in the upper tank of the radiator and had it fully serviced and repaired by a shop that is next door to a British Car mechanic, and he knows his stuff.
 
Friday the car ran perfectly taking back roads across the valley into the foothills to Mariposa, the official start of the rally. Saturday while zooming up and down hills, chasing and being chased by 60 participates in 1975 and older cars, the radiator temp got up to 210. The radiator cap would then loosen up on it's own about 1/4 turn, release pressure and lowering the boiling point, and the radiator would boil over.  After the first time my son fashioned a capture bottle out of a water bottle. Next time it happened the bottle filled up, but with the cap getting 1/4 turn loose, when we'd get over a pass and going down the other side and the temps going down, the coolant wasn't being sucked back into the radiator.  After the third time we took the thermostat out to reduce any restriction, and that seemed to help a little.  Sunday, even though it was hotter, we only had it happen once.
 
If 220 is enough to cause a boil over, I’d think the capture bottle would fill up but not unscrew the cap. Then when it cools down it would suck the fluid back into the radiator. I think it’s the radiator cap and plan to replace it.
 
Thoughts?
 
Rye

PS: If you want to see photos of the weekend, here is a link https://ttscc.shutterfly.com/pictures/3466
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