Smell coming from air/ circulation

I recently heard discussion about a sweet/ unpleasant odor coming from using heat / air and I have been having reoccurring problem and taken it in many times and they find nothing. It typically happens in winter with heat/defrost especially if in recycled air. They have checked all filters etc. It has been bothersome since I bought the car. Many times repair says they cannot reproduce the smell and tell me to bring it in when it is happening! Like that can happen!

If it is a sweet smell, that is likely a heater core leak. You can do some detective work yourself by monitoring the level of coolant in radiator every few days. (Be sure to check when the engine is totally cold!) If you see the coolant level dropping, that would confirm the probability of a heater core leak.

While this will likely be a very expensive repair (relatively cheap part, very costly labor), this is not something that can be deferred, simply because you and your passengers are breathing aerosolized coolant. Take it from someone who learned the hard way–breathing this for an extended period of time can result in pneumonia.

If it is not sweet smelling, maybe even musty, it may be all those leaves and things that fell through those vents right in front of the windscreen. If it is working as it should, you can pour some water in there and it should come out under the car. If it does not clean out what you can and you may need to clean the drain hose as well. That hose usually comes out under the car right in front of where the passenger puts their feet. Use a LITTLE compress air or some string trimmer line.

VCD: could you expound on this in your post above?—> “you and your passengers are breathing aerosolized coolant”.
I have been liveing with a heater core leak for some time. I never smell it but my passengers do, or think they do. The leak must be incredibly minute because I have never seen coolant drip out the heater box, and my coolant level never goes down more that what normal evaporation out of the resivoir tank would be.
How serious a health issue is this? Is there something like the ethelene glycol, or propeylene glycol that is really dangerous to breathe? I knew it was poisonous for all living things to drink but never knew about breathing it. Thanks for any info.

According to the EPA, besides the well-publicized problems that result from drinking Ethylene Glycol, it can also be absorbed in toxic amounts through the skin and by breathing its vapors.

I can’t give you any information on the process by which it causes lung problems, but I can tell you that after driving my '86 Taurus for a few months with a leaking heater core, I fell ill with pneumonia. Following my recovery from pneumonia, I was diagnosed with asthma. Prior to my exposure to the leaking heater core, I had never experienced any lung problems.

OK I guess I will be driveing with my windows down untill I can replace mine. Thanks for the info.