I was bleeding my 1996 328i today after I drained the radiator plug and put in new coolant. I let the car run for 15 mins, then the expansion tank was boiling the coolant while the bleeder cap was slightly opened. I turned the car off and then I checked the oil and it looked very normal. There is also no leaks at all and the exhaust is not bringing any smoke out. (I am using 50/50 BMW Coolant). LMK what the problem is or could be. Thanks