I had the same problem. Shaved the head, new head gasket, new water pump. Nothing worked. I ended up just buying a new drop in motor. But as I thought about it, I never checked to see if the water pump was actually turning. I am guessing that the chain that drives the oil pump and water pump spun off. If I ever find the time I am going to disassemble the old motor and find out. 2018 750 challenger crew.