What causes oil in the radiator? Got it all back together and noticed that the valve cover and exhaust manifold was very hot. Then the over flow tank started to bubble. I just happened to look inside and noticed there was oil in it. I drained the radiator while ago and there was a good bit of I\oil in it. I drained the oil out of the motor and it seems to be fine. Im lost and need to get it fixed ASAP.