Losing coolant/ no leaks

if there are no visible leaks, you might have a bad head gasket. you can do a combustion leak test yourself by getting a kit at the auto parts store for about 50 bucks, or go to a shop that has a "sniffer" that they can check it with.

check your oil, does it have foam in it? If it does then your engine needs replacement. Your antifreeze is leaking inside your engine.