your head might be going out
Can be a hole in the radiator or a problem with one or more of the hoses. A mechanic can run a pressure test to see where the problem(s) are located at.
You need to pressure check the coolant system to determine just what is leaking first and the address that problem..........
Have system pressure tested at local shop unless the problem is obvious. Check coolant lines (upper and lower) to see if they are leaking. Check radiator for leaks. Water pump may be leaking. Also head gasket could be letting coolant in to oil. Pressure testing will check all these components
If only adding coolant to a leaking system it is safe because soon you will be forced to drain the coolant and fix a problem
You have a leak in the cooling system. If you don't see evidence of coolant leaking on the ground, you probably have a head gasket problem. Have the system pressure tested soon before more serious damage is done.
Is it too full? Well some vehicles are equiped with a little hose that will let coolant out if there is too much or too much pressure, but not all vehicles will have it. (If you are talking about the overflow tank, if the radiator cap is leaking, check to see if the cap is in good condition)
If it is an automatic transmission, the cooler in the radiator is most likely leaking into the coolant system.
It can run but if it is leaking coolant it will cause a problem soon.
A blown head gasket can allow coolant to see into the manifold, and out the exhaust. A broken piston ring can also cause this problem.
Just because there is a leak doesn't mean it will overheat instantly, but if you do not keep adding coolant or fix the leak you will have engine problem
If coolant isn't leaking externally then it has to be leaking internally. A leaking head gasket or cracked head may be the problem. If this is the case, as it gets worse more coolant will enter the combustion chamber or engine oil. Either way you will be able to determine as it gets worse. There will be white smoke from the exhaust. May be intermittent. This is if coolant is leaking into combustion chamber. If leaking into oil, you will notice that the engine oil will have a brown/milky look to it as you check the oil level on the engine oil dipstick.
Where is it leaking? That pressure test sounds funny, why would you pressurize only the pump and radiator? If you are leaking coolant outside the engine, locate the source. If you are losing it from the radiator but cannot find a puddle suspect a head gasket problem. Check the Radiator cap first, if it is bad high pressure coolant will travel back to the overflow bottle.