Ford Repair: coolant loss, coolant line, coolant loss

Question
Hello, I have a 2000 Ford Explorer with a 4.0L engine. It started to overheat, the gauge was moving around, no heat, it was blowing cold and I quickly realized the coolant level in the radiator was low. I filled it as much as I could and now I don't see any leaks externally whether from the water pump weep hole or the thermostat housing or any coolant line or inside on the passenger side below the heater core. As I was filling the radiator up I noticed bubbles coming out of the inlet to the overflow jar. After running the vehicle a short time, maybe 10 minutes, the bubbles stopped so I assume the bubbles were from the low coolant because the level was still low in the radiator. I am out of coolant and do not want to put water in as it is cold here, winter time. To go back to before, I filled the radiator halfway with all the coolant I had and saw no external leaks from anywhere. Still no heat when heating fan is on. I'm kind of dead in the water, but wondered do you have any ideas as to what to check for next or am I basically stuck until I fill it up? I get the feeling that even when I fill it, I won't see any external leakage. So if that is the case, then do I consider a head gasket? The vehicle only has 110K on it. Any possibilities that I might not have considered? Thank you.

Answer
You need to get the system full to eliminate air pockets, and pressure test to find any leaks. The lack of heat from the heater could be just air in the cooling system, although Explorers have many issues with the temp blend door.

Be careful when adding antifreeze. It should be mixed 50/50 with water; if you end up with too strong a mix with too much antifreeze this can have the opposite effect and cause overheating, because antifreeze does not transfer heat as well as the mixture at 50/50.