Hello fellow Volvo lovers :)
I'm new to the forum, having recently acquired a 1989 740 Turbo (w/ about 170K miles), and am having some problems with what I hope is NOT the heater core. Unfortunately, the car has been exhibiting some of the telltale signs of a heater core leak (ie: mist from the air vents, fogged windows, leak under passenger console) and, earlier today, the heater stopped...well....heating. My question is: How can you differentiate between a control valve leak and an actual heater core leak? If the leak is in the heater core, is there any "temporary fix" for it, to just get some heat into the cabin without having to rip the console apart just yet? I've heard mixed reviews of some of the commercial "stop leak" agents...Would it be advised that I try one of these, or would it not do any good, considering the heater has already stopped blowing hot air?
You'll have to excuse me if my questions seem extremely novice, but, although I am pretty good at diagnosing what's wrong with a car, I lack the experience to actually attempt to fix something like this myself (especially since I'm a girl. lol). Anyway...Any help or advice is greatly appreciated. thanks in advance!

The earlier advice is spot-on. I am not sure about the temporary "fix" (disconnecting the hoses at the firewall and inserting a pipe with clamps to route coolant away from the core) since every time I have removed these hoses, they've had to be cut away. When you do replace the core, change both heater hoses and the heater water valve as well. The valve is, at thirteen years of age, close to failure anyway. See the FAQ for more information.

Thanks for the great advice everyone. The saga continues, unfortunately, in that it looks like there's a problem with the thermostat as well. The car now begins to overheat when driven over approx. 10 miles. Needless to say, I believe this has become a case for my mechanic :P Do these failures usually coincide, or was I just "lucky" to have all this happen at once? What a day! :)

The heater valve I bought from Brickboard sponsor FCPGroton is a metal unit, not plastic, like the Volvo original.

Presumably this will avoid the catastropic coolant loss failure of the plastic units.

While replacing mine, I briefly considered hoe to inject "stop Leak" just into the heater core and not into the engine block.

With the heat control valve and/or the hoses removed it is easy enough to rig some connection to the inlet/outlet pipes of the heater core at the firewall.

I couldn't figure out what to use to pump hot water and the stop leak into it, so, I didn't bother.

With the hoses removed, I did give the core a good back-flushing, however.
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1989 740Ti 1986 240DL 1984 244GL

You have a leaking heater core. A control valve leak would leak in the engine compartment. The only possible leak inside the cabin is the heater core. There are no hoses, clamps, valves inside. Your symptoms are exactly what mine were: mist from the air vents, fogged windows, leak under passenger console. I tryed a couple of different stop leak products, they worked for about two weeks and then the leak became much worse. Sorry to tell you this, but that core is going to have to be replaced. By the parts online.
I don't have an answer for why you are not getting heat- I connected the heater hoses together at the firewall (bypassing the core) until I could get it replaced. Have you looked at the heater valve to see if it's opening? You could check it by applying vacuum. Good luck.