Not all coolant smells sweet when heated.... the more common ones do though (primarily the green stuff).
Could you be more descriptive of the smell? Does it smell like sulphur? You may be experiencing multiple issues and a BHG isn't out of the question. I'd start with testing the coolant pH and a compression test.
My money's on a BHG. It's a 7M afterall, I've got a 8 out of 10 chance of being right... easy money.