Thanks so much everyone for your insight. I replaced the cap today and went on a quick but hard drive. No spewing! Cautiously optimistic replacing the cap fixed the issue, although today's weather was about 25 F cooler. I'll keep an eye on it and keep you all posted.
By the way, does this look like it's full but not over full? The diagram is a tad cryptic to me, as I'm not sure what it considers to be the "top" that the bobber protrudes past. This is with the car stone cold.
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