So, after a long rain this weekend, and my car sat for about a day, I started it up and began driving about 20 miles back to my apartment. after roughly 5, this steam started pouring into my cabin, and i heard some sizzling behind my dash. I stupidly thought it was rainwater boiling back there, because my car is pretty leaky. it got more and more intense, and when i made it back to my apartment, i noticed my temp gauge had shot all the way up, and then POW! the cap blew off my reservoir tank and i had this little volcano on my hands. My radiator looks totally normal.



what's going on?

 

I'd say the thermostat is stuck closed, or the radiator is blocked up, or the water pump isn't freely spinning (rusted up / loose fan belt).

Hopefully you didn't toast your head gasket.

 

would blowing the HG be a result of the above things happening or a cause?

I'll check those.

 

A blown head gasket can definitely make the coolant blow... but a blown head gasket is usually the result of a cooling system failure (on the n/a), or some monster cylinder detonation / misfiring.

I overheated my Geo Metro a few years back (Driving 80mph in 105*F with the a/c on ... for a 3cylinder that is absolutely maxed out). It started puking coolant out the over flow, and even after refilling it and changing the thermostat & cap, it would still throw coolant. It was a hole in the head gasket pushing the cylinder compression (air) into the cooling system until the weakest point would fail (plastic overflow cap).

Click for a good read: https://www.zdriver.com/forums/other...g-metro-29223/