Kia Troubleshooting Questions? Ask a Mechanic
Hello I will help you with your question,
That is the valve you open to drain the coolant from the radiator.
It is a plastic nut you screw in, if yours is gone look for an orange plastic piece with an oring and a slot in it. Maybe it never was replaced after draining. The dealer will have one, I had our parts department stock some as they break.
If you don't have a KIA dealer close try Hyundai.
Let me know how I can help
Could just be the radiator has a crack also if your mechanic never saw the radiator and just answered a question. Pour water through and see where it is coming out.
Let me know what I can do to help