I have a 62' Willy's Jeep Wagon. The heat is terrible so I have been picking at the causes. I pulled off my radiator hose, and thermostat (where it bolts to the block) and it was all backed up with anit-freeze. Inside the block. So full to a point where is began to sit on the valve cover. I am going to replace the thermostat as I have a new one in hand, but would anyone know why that is happening, or is that common?