My '94 940, 7 months ago, had a heater core failure. I noticed it when the car overheated. My mechanic replaced it and, upon its return, whenever I turned on the heater (with or without the fan) a horrible acrid smell resulted (not antifreeze-like, or mold but more oil-based). This had not occurred previously. When I turned on the fan after the new core was put in small bits of insulation came up through the ducts, for several months! I brought it back after 3 months, as the smell had not at all abated, and the mechanic went back inside to look for clues. He put a heat gun onto a bit of the insulation and it had a similar smell to the one that I had smelled. Anyway, he reassembled the heater, and the smell was still there. I was very frustrated as I had paid quite a bit to have the heater fixed, and it was too smelly to use. I decided to take it to another mechanic 3 months later (6 months after the initial fix), and brought it in for him to smell (he agreed that it was not antifreeze). Two days before the appointment the windshield fogged up and I smelled antifreeze. When I brought it in, indeed, the new heater core had failed. I had that replaced, and the offensive smell reappeared. I've tried, in both instances, running the heater on full blast for hours at a time, in order to dry out something. It has not worked, and over time the smell has not abated. The heater now does not provide as much heat as the original heater did...I don't know if this is related. Both mechanics and me are completely stumped as to the cause of this; it does make the heater unusable though. Does anybody have any ideas?