Heat smells but told heater core is OK - help

Hi. My 97 Odyssey (200,000K) stinks when the heat is on - smells chemical-like, and a bit sweet. Plus there is lots of moisture inside. I know, I know, it sounds like a classic heater core leak, but I have had two mechanics check it and they say the heater core is FINE. (These are good mechanics we have used for years.) We had it checked again for a third time today (by mechanic #2 - the better one). He says the heating system works fine. But if it smells so bad you don’t want to turn it on, its NOT working. He says there is nothing he can do about the smell. What to do?

It is possible your mechanics are misdiagnosing the problem, but I’m wondering if the AC condensation drain has clogged up. You likely aren’t using AC now, but when you turn on the defrost the AC may kick in, even though you don’t know it. Ask your shop to check that drain, make sure it is clear.

This wouldn’t seem to explain the sweet odor you are smelling, that sounds more like a coolant odor, but it is possible some combination of mold & carpet decay from simple water accumulation could mimic a sweet odor. The heater blower is simply kicking the odor up from the floor area, so you notice it more with the heater on.

It’s also possible rain and melted snow is entering the passenger compartment somehow or the other. Is it wet anywhere under the carpet? If so, check all the drain holes at the bottom of the doors and unclog any that are not open. And figure out how the water that enters the vents under the windshield is designed to drain out, and make sure that path is free and working.

A 1997 van, I wouldn’t be surpised if the heater core is indeed leaking, in spite of the mechanics being convinced it’s okay

I wonder how they determined it’s okay

Pressure test?

The symptoms seem to point to a bad heater core

Have you been getting a slippery, hazy film on the inside of your windows, especially the front window?

Thanks for the ideas. FYI: both mechanics did a pressure test.