So yesterday my car decided to overheat ... during city stop/go driving on a 32C day while driving somewhat agressively. There was bubbling/boiling coolant in my overflow tank, and a loss (from overflow) of coolant in my system - about 2L in total that I had to refill once it cooled.
Trying to diagnose the issue ... The lower rad hose starts to get fluid moving when my temp reads about 85C ... so I don't think it's the thermostat (which was replaced last summer, about 6,000km ago). The radiator fan seems to be engaging where/when you expect it to ... If I have the engine off but turn the ignition on (while hot) fan kicks in right away, so relays, etc. seem to work fine. Also, there's no leaks found under car EXCEPT that one time yesterday when it was boiling over.
Here's why I'm wondering about the head gasket ... I had coolant in my overflow today after a test drive, (though not boiling in the overflow) which did NOT get sucked back in while cool. Also, my oil dipstick is pretty brown (see vid link, below), yet I only have about 500km on a fresh oil change of 5w30 Castrol Edge Extended-Performance full synthetic oil - so I find the colour suspicious. (Though truth be told, I seldom look at my dipstick within the first week or so after an oil change, so can't tell you what it normally looks like at 500km.)
That being said, the coolant in my rad is PRISTINE. Not a spec of contamination in the rad, hoses or on the cap. Also, there is no white smoke whatsoever, so I am somewhat confused. Does anyone have any suggestions as what else to check? I'm going to throw a new radiator cap on and see if that makes any difference, but I'm perplexed...
FYI EP82 4E-FTE, mostly stock except with WEPR manifold/downpipe and running 12psi boost. Generic 2-core honda aluminum racing rad.