Replacing the heater core is extremely time consuming. You basically have to remove most of the dashboard.
That said, the next thing I should say is do NOT use any form of stop leak. Those products cause all sorts of problems. In addition to maybe (but probably not) clogging the leak itself, they also clog the rest of the cooling system. Very bad, not unlike clogged arteries of a person. You don't want that stuff inside the cooling jackets of your engine and elsewhere.
Secondly, just because you're getting a whiff of coolant when you turn on the heater, it doesn't necessarily mean that it's due to the heater core. If there is only a little coolant leak under the hood, the blower motor will suck it into the cabin for you to smell. So it's hard to say where your leak is coming from. It could be the heater core (especially if accompanied by wet footwells near the heater box and a persistent oily residue on the inside of the windows), but it could also very likely be some other cooling component under the hood. Either way, you have a leak which must be fixed. Time to start looking around with a flashlight.
1998 BMW 328is
1966 Pontiac GTO
2007 Subaru Impreza 2.5i 5-door
View my photos: Caught in the Wild
Last edited by ZeGerman; 01-30-2013 at 08:12 AM.