Going to work on the freeway, cruising around 65-70mph. Check engine light came on, but I've been conditioned to not think too much of it since it's always P0420 from me relocating an O2 sensor.
Drove on about 10 more minutes after that on the freeway, with no audible/visible concerns. Then felt a brief loss of power, dashboard went up like a christmas tree, then a total loss of power and pulled over. Popped the hood to see the coolant expansion tank cap popped open along with a nice explosion of coolant, some of which had this weird almost congealed slimy texture to it (have no clue how this occurred? from overheat? or contamination?). Temp gauge was pinned at the top at this point. Car was allowed to cool down about an hour while the tow truck came. Was not able to start again to drive it onto the flatbed.
Analysis after getting home revealed the spring-loaded portion of the radiator cap did not seal at all to the inner lip of the radiator any more (also no clue how this happened?). But this seems a possibility to be linked to the recent slow loss of coolant I've been experiencing. Spark is still present. I can still hear the fuel pump prime for a second or two when keying ON. The car cranks, but doesn't show any attempt to start. Haven't had time to do any more investigation yet, but hopefully will over the weekend. Let me know what yall think