I am working on a very neglected 2004 325xi. It runs great but...I pulled the valve cover to replace the gasket and it was completely coated with dried up oil sludge to where there was almost no bronze color of the cams showing. I don't know how oil was even getting around in this thing. I carefully loosened and sucked out most of the bigger sludge and also cleaned the inside valve cover but the head is still pretty black and tarry looking. Is there a way to clean it without disassembling it? Can I drain the oil, leave the pan plug open and then flush the head with something till it’s clean and then refill it with fresh oil? Is that a bad idea?