no not always will you get the sweet smell of coolant the heater core could just be full of crap and not letting coolant get through it. What I would do is take the coolant return form the heater core off where it goes into the block (or intake i cant rememer exactly where it goes), plug the hole in the block with something (A small piece of hose with a big bolt stuck in the end should do) and hold the hose over something to catch any fluid and start the car. if fluid comes out of the hose you are good and this isnt your problem and you have wasted 20 mins but if it doesnt or it comes out slowly then your heater core is bad.