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Why does smell come off vents in car under certain conditions?

I’ve noticed the following and wonder what might the reason be for it:

  • A/C running and vents open - no problem with smell/odor
  • A/C off, heat/fan off, vents open - sometimes get this engine-y smell coming out of vents
  • Heat fan on, vents open - usually no problem, but sometimes may get engine-y smell out of vents

The smell mostly comes when vents open & fan not operating. And definitely no smell when operating under A/C mode. I just changed a dirty cabin air filter. So we’ll see if that helps. Could it be dirty filter? Something up with ventilation system or the blower fan?

Some vehicle AC systems emit a strange odor just after you shut off the AC in hot weather. I’ve noticed this over the past 40 years and have never found the reason for it. I have also smelled this same odor in some freezers so it has something to do with refrigeration. Maybe someone here can clear up the mystery.

Consider this: Mold in the plenum - perhaps due to a tiny coolant leak around the heater core, or AC evaporator due to a small refrigerant leak. When a large volume of air is moving, the odor is diluted and not detectable. Whenever the blower is turned off, the air flow diminishes and allows the odor to become concentrated and thus more noticeable. All it takes is a few drops of moisture to allow mold to grow. A possible temporary fix is to spray a generous amount of fungicide (such as Mold-X or even Lysol) into the air intake while the blower is on full. See if that makes a difference.

The engine smell might be from an oil leak.

When you set the vent system to AC it goes into the recirculation mode. This means instead of drawing outside air into the passenger cabin the air inside the passenger cabin is recirculated. When you set the vent system for heat it draws outside air into the passenger cabin. If there’s something burning on the engine such as oil, this burnt oil smell gets drawn into the vent system thru the openings at the base of the windshield.

Check to see if there’s something burning on the hot engine.


@Tester, I though recirculation mode was an option, but not by default used for A/C. Most modern cars have a recirculation mode button or so that you can enable or not. Is that separate from what you are talking about? Or do you mean when using A/C it makes no difference if you have that button on or off?

FYI, got tired of the smell and might be allergic to it, and a while back noticed A/C duct/vent cleaner for cars. I used that following its instructions, and that seems to have helped. Definitely smells better when I’m running it on fan mode only, or entirely off. Guess it may have been prematurely dirty A/C duct & vents.