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Strange odors when the temperature is below freezing or snowing

I moved my 2001 Subaru Legacy from Seattle WA to Denver CO last fall. Even with 95,000 miles on it and fully loaded the car made the trip without a problem. When the first snow fall and below freezing temperatures arrived in October a strong odor that I can best describe as a mixture of exhaust and antifreeze was present in the passenger compartment. I checked the level of the antifreeze, but it remained constant, so I am assuming no leak. This only happens when the temperatures are below freezing or it is snowing. Anyone have any ideas?

Could be a slight head gasket leak that only happens when the engine is really cold.

I would watch the coolant level and maybe inspect the spark plugs.

A customer relating to me that they smelled antifreeze inside the passenger compartment would direct me to the usual suspect (heater core) I would not know how to modify my search by the extra info that the smell was only present at below freezing or snowing (above freezing) temperatures, the search would be the same.

Now if this was an issue at -40 things could be different

Thanks, I have been watching the coolant levels, but I will pull the plugs to see if they show signs of burning anything but gas.

would you recommend replacing the heater core to be safe?

No, never would or should you replace a heater core without a very thorough investigation. I don’t know the degree of difficulty with a Subaru but typicaly these are big dollar jobs.

Can you feel a “film” on the inside window glass? this is a sign of coolant getting into the passenger compartment some way, if you still want to ge better confirmation on the heater core, pressure test the system and see if some type of panel can be removed to get a better view of the evaporator housing (also contains the heater core).

Do not replace the heater core unless 100% postive proof is given it is leaking

I don’t think temperature is the issue, but the weather might be. Whether above or below freezing, if you’re running the heater (below freezing or snowing) then coolant is running through the heater core, giving it a chance to leak inside the car if the heater core has a leak. If the heater button isn’t pushed, (if it’s like older Subies) the coolant will be shut off to the heater core and redirected back through the cooling system. That leaves coolant in the heater core but not under pressure as it cools down and perhaps not leaking as a result.

As oldschool advised, I wouldn’t mess with the heater core unless you’re sure that’s your problem. Even on older Subarus, it’s a very involved repair that requires alot of your patience (as the mechanic) or alot of your money (for a different mechanic) even on a “simple” system. If you have a “climate control” feature, it’s probably much worse as it will likely involve the AC as well. Tread lightly.

Again, as oldschool noted, check for a “film” on the glass. Or “fog” on the glass that your defroster isn’t able to remove.

Thanks to you and Oldschool. I’ll have to check this out more thoroughly.

Antifreeze from the heater core would be my first suspect, but I do want to add the possibility of winter mix windshield wash fluid vapors. When the weather is bad and I use mine a lot the smell will sometimes work its way int the passenger cabin.