It could be any of the things you mentioned, but the clue is you smell it when you turn on the heater. One test that can help is a pressure test. You might be able to borrow or rent a tester from an auto parts store that borrows tools. Pump the system up to 15 psi at the radiator, then watch to see if you can identify where it's leaking. If it is the heater core, it will drip into the condensate drain pan, (if the car came with air conditioning), and will drip onto the ground under the area of the front passenger's feet. You can check at the end of the radiator and heater hoses to see if a clamp is loose but you typically won't smell that inside the car. Normally leakage from the heater core will cause fogging and a greasy residue on the windshield when you use the defrost mode. If you smell it but it doesn't cause fogging, I would be looking more for something leaking under the hood, and the smell is being drawn in through the fresh air ducts.
One problem with heater cores too is sometimes they develop a leak due to corrosion but the leak is blocked by the sediment, (sand), that never gets fully cleaned out when the engine block was cast. During a pressure test, that sediment can actually help temporarily seal the leak. Very often the leak will show up right after the cooling system is flushed and cleaned.
Another test that will probably be less effective for this problem is to add a small bottle of dye in the radiator. After driving long enough to warm the engine, you search with a black light. The dye will show up as a bright yellow stain that you can follow to the source of the leak.
Friday, April 29th, 2011 AT 4:48 AM