White smoke/rough start only sometimes

white smoke and rough start but it only happens sometimes. so far only after its been sitting for hours. replaced thermostat and radiator cap.
Started normal today, let run maybe 3 minutes and the coolant already overflowing out of cap but no white smoke.....
seems odd it only happen sometimes. any ideas?

 

best case: bad head gasket
med case: gasket and head
worst case: cracked block

coolant being pushed out is not a good sign

 

it ran great for like a week and a half after doing those two small things and now back to only running good sometimes. would a pressure test tell me if its the head gasket?

 

a compression test will tell you how much pressure a cylinder generates when turning over or can hold if using outside air
a leakdown test will tell you how quickly a cylinder released pressure
nether test can tell what part(s) is/are leaking

 

What if there isn't any coolant in my oil or vise versa? It's not leaking on the ground either. Sorry I'm definitely not well versed on mechanics

 

those are good signs but not proof, unfortunately a visual inspection is going to be called for to make sure

I boiled an engine dry one time in my youth, after putting water back into it and trying to start it the escaping compression gasses blew the overflow tank clean off. Sounds like a failing gasket, so go in hoping for that be be prepared for the worst.

 

White smoke could be gasoline. Next time it puffs get a wiff, if it is coolant it will smell hella sweat. Gasoline, not so much.

 

Stick a fork in it it's done. Sounds like head gaskets at least. You can rent a head gasket testing tool for autozone to be sure. Heres a YouTube video on how to use the tool.
https://youtu.be/U0DdVQoBu4U

 

Thank you for all the advice!

 

If you're gonna pull the head, have it rebuilt if financially practical. Taking it on and off sucks. If I ever have to mess with that head again, I'm swapping the entire motor. I bet in half the time.