In the past six months i have replaced header tank ,water pump intercooler radiator .today i watched it drained ,then i got my coolant warning ,i pulled over it was empty .i undid the cap it refilled instantly ,i got home took cap off again while coolant was still in then fast idled with everything turned on heater etc it drained in about 20 mins and never came back lol i looked under car no visible leaks so i topped it up again .once the engine was up to temp again i left the lid off and the coolant was just overflowing the neck it seemed to burp lol and the coolant level dropped but then slowly rose again until it burped a few more bubbles .it sounds like its under a lot of pressure when it rus hees back into tank .im probs not making sense lol but i can only frscribe it as i see it i have no technical terms .