My Pontiac Grandam

You could have a coolant cap not holding the correct pressure and venting the coolant off as steam or a leak somewhere without checking it can't say for sure.If you have never replaced the coolant cap i would.Also rent a coolant pressure tester its a pump with adapters that you use to pressurize your cooling system to find.Try auto zone they rent tools you can try other places too.let me know what you find.

I should elaborate on what I've already done so far. I replaced the cap a couple years ago, maybe 3. I also noticed that my cooling fans were not working so the first thing I did was to check the fuses under the hood. the 2 fuses for the fans are still in working order. I did replace the relay for the fans after noticing that the fans would come on when I turn the air conditioning on, I also replaced the temperature sending unit. Now it seems like the fans don't come on at all wheather I turn the air conditioning on or not. When a person turns air on, should the fans come on instantly? or after some amount of time?

Another couple questions. Is the best time to add coolant when the engine is running and at temperature? While the engine is running should there still be coolant in the fill tank? up to the "coolant hot" line?

If you replaced your cap 2 or 3 years a go you can't say its new and isn't bad.I would just change it there not that expensive and you have no way of testing it.It looks like the fans come on with the a/c and when the engine reaches a certain temp usually about 221 degrees.I posted a fan operation for you i would give the fan a little time to turn on with the a/c the fan comes on to keep the a/c pressure's down and to help turn the r134 from a gas to a liquid.As far as the coolant level i would check it or add it when cold so you don't burn yourself.Also yes there should be coolant in the bottle when its hot.