Question: I've notice over the course of the last week or so, I have checked in the radiator to make sure there is antifreeze. Even when I take the cap off when the car has been sitting for a long time to when the engine is cold, antifreeze will spew out, but there would be coolant left. Today I took off the cap and I did not see any coolant, so I added more. However, the reservoir tank was full (overfull, actually). I haven't noticed coolant in my engine and I don't see oil mixed in with the coolant.
Answer: We really need to know the make and model and year vehicle you are talking about. Certain years of engines have certain things that cause the problem you're talking about. But in general I can give you these ideas.
Pressure in the cooling system even when it's cold is never a good thing. You have an internal engine problem. Most likely a blown head gasket. Now, I know you say you don't see any coolant and oil mixing. But that is not always the case with a blown head gasket. Most people think that's the only way to tell a blown head gasket is oil in the coolant or vice versa. But that is not the case.
Again, depending on the engine, a head gasket can be blown in such a way that the compression from the cylinder pumps pressure into the cooling system. Internal cylinder pressures run hundreds of PSI, the cooling system and radiator only run at 16 PSI. Therefore compression gas from the cylinder is getting into the coolant, antifreeze would not go the other direction into the oil.
A crack in a cylinder head will do the same thing. This can cause the engine to burn coolant, not contaminate the oil, cause excessive pressure in the cooling system and fill the overflow jug too high. The first thing I would do is remove all the spark plugs and look for signs of burning antifreeze. If it is the newer coolant you would see orange or yellow deposits. If it is an older car with the green coolant then you would see green deposits on the spark plugs. You may even find a spark plug electrode that is completely burned off.
Next you would need to do a cooling system pressure test. This will show any external leaks and if a blown head gasket or cracks of a cylinder head is the problem it may fill up that cylinder with coolant because the engine is not running and the pressure will push coolant into the cylinder.
Antifreeze Low And Blows Out Reservoir Tank