My 2003 4.7L started overheating and obviously was losing coolant somewhere. After adding coolant, the thermostat wouldn't open until it almost red-lined the guage. Today I drove to autozone and back no problems. Changed the thermostat, added coolant, went to fire it up and it started to crank but wouldn't turn over....??? Heard a grinding noise, so I took serpentine belt off and tried once more but still wouldn't crank. I have no idea what I just messed up when changing the thermostat. Crank shaft sensor? I would greatly appreciate any ideas, thoughts, assistance. Thanks everyone.