Under the thermostat housing mounted on the engine at the "engine side" of the upper radiator hose. That "usually" happens when the thermos is stuck open & the computer is getting cool readings from the anti freeze because its flowing too freely. More fuel is added due to bad temp readings. Drain some coolant, remove rad cap, remove thermos housing, Clean gasket or r/r O ring & replace thermostat. Reinstall. Refill cooling system, let it run.refill overflow bottle after warmed up. Ck for heat & leaks. Good luck