10007 Leaking Heater Core
If you're starting to smell a sweet odor, seeing steam out the vents, or you’re experiencing a sticky film on the windshield chances are your heater core is leaking. A leaking heater core can also make it very difficult to clear your windows of fog or condensation since it's blowing moist air out of the vents.
A leaking heater core can also be tough to diagnose if the above symptoms don’t show up but your coolant is always low with no signs of external leaks.
In some cars, the leaking coolant can get underneath the carpet and you won't see any visible damp signs due to the waterproofing on the back side of the factory carpet. This means heater cores can leak for quite a while before you might notice it in the interior. If you are finding your coolant level always seems to be low but can’t find any external leaks take a close look at your heater core, it might just be your culprit.
Early Volvo models used all metal copper and brass heater cores. These cores last for a long time if the cooling system is in good shape and the coolant is flushed periodically.
Volvo started use plastic and aluminum heater cores in the 850 and newer models. These cores are prone to leaking where the glue or seals fail where the plastic meets the aluminum. It would seem logical that repeated expansion and contraction of the plastic would cause seal failure long before the core itself actually failed .