1993 Toyota Corolla Heater does not get hot

The heater in my car has not been getting hot for a while now. Well since the dealer told me that the problem could be in the aftermarket radiator cap not allowing the proper pressure to build in the system i think i know what my problem is. My car has been burning coolant. There's no leaks and you can smell it and hear it (rough idleing) durning start up. I'm figuring my pressure is being lost there. The problem is though i dont know if it is in the head or the engine block. I dont want to spend the money on a head gasket kit just to get it all back together and it still isnt fixed because of a crack in the block or something. So my questions are: Which is more likely to be the problem, head gasket (or the head itself) or a bad block? And could that be the reason why i'm not getting heat?
Thanks, JC