Okay, so I think I've been getting some telltale signs. There is a slightly sweet smell when the heat is on, but it comes and goes. I'm guessing this is the smell of coolant. The windows fog up sometimes as well.

Tonight, all my windows were fogged. The only patches that weren't fogged was the front right near the windshield defogger vents. Driving like this is becoming dangerous. I'm not sure if it was the coolant leak causing all the windows to go foggy though. My socks got wet tonight while working on some stuff so that might be it, but seriously, a pair of socks shouldn't fog up every single window. By the way, it's very cold here at the moment. About 25 degree F.

The problem is I have no idea how to go about locating this coolant leak. I don't seem to be losing it very fast. I was down to about the Min line in the reservoir yesterday and that was after having topped it up about 3 weeks ago, so it's dropping pretty slowly. The heat is warm when it's up all the way, not hot. But when I drive on the highway a bit, it seems to get hotter.

Any ideas where I should start? Tomorrow during the daytime I will check to see if any of the carpetting is wet, but that may also simply be because of some snow having been tracked into the mats and stuff and not having dried out since the weather has been below freezing lately.

Thanks!
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Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'

someone mentioned that the valve might be bad or something. i'm planning on ordering the parts today from FCP, anything else i might need besides the core? new valve?

thanks.
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Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'

Although the plastic heater valve is in the engine compartent and will not leak into the passenger compartment, it should be replaced as routine maintenance. Just ask one us us who has had the valve break while driving. That is why the
Coolant Loss Sensor is such a good idea.

You can bypass the core by connecting the two hoses leading to it together. That way no more coolant gets in your cabin and on your windows. I did that to mine to get me through until I had time to deal with it. (In colder climates heated seats are great for this....)

-Eric
'89 765t, 175k, with an '02 heater core installed after many long hours of 'fun'.

You have a heater core leak. It is the worst maintenance job on a 700 series car, and worse than the heater motor on a 200 series.

I will be replacing the heater core in my 745T this week for the same reason and I will take pictures and do a post afterwards.

Per Steve Ringlee's suggestion I used some Bar's Leak in the cooling system about 5 weeks ago. It has almost completely stopped the leak. I haven't had to add any coolant in these 5 weeks, and it was once a week prior to adding the Bar's Leak. I recommend the stuff highly. As far as I am concerned it is nsot a permanent repair, but a great stopgap measure.

Great, thanks John! I'll use that Bar's stuff until the weather warms up or something or when I can find time to get this done. First, the UV dye and light... :)
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Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'

Invar
I'm going through the same heater core r&r I know now why the shops get $700.us to do the job...wow ! You can really make em bleed by running the revs way up, like your passing someone. Or in a drag race ! the trouble is... it only gets worse and can blow any time. Look for my post earlier "heater core exchange"
for a few details. It can be done in a day just order the parts before you dig in, make sure you have the right hoses , new clamps, etc. You won't believe what you have to do to get the cover off the distribution unit !!If you have had the center console, radio out etc, etc. it should be a cake walk.
Let me know through the board any tips or ? that you need, have, as I am about to reinstall the core back into my '87 745.
It is in place, but I got called away , so had to put it on hold.
Back at it tomorrow to hook it up then I can work with heat in the car anyway.
Pete
Portsmouth,NH

Yikes... Well, I think I can tackle this job, it doesn't sound TOO bad. I've had the console and fusebox and radio and A/C unit out before so it'll just be like that, except all at once. :)

In any case, I think I will do what alchemist has suggested and put some UV dye into the coolant and look for the leak with the UV light, that appears to be the best idea. Man, this really sucks. I just dropped a chunk of money on the car not too long ago for the rear rotors and front bushings. BOOO. :-)

I'll keep you (and everyone) posted. I may take a while to get around to this unfortunately, but it's unavoidable. If I had the cash and ability to do this immediately, I would since I don't like driving around with a coolant leak or anything as it is.
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Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'

The only place coolant can come from in your car is the heater core. The lines are metal through the firewall in the air conditioned car and there are no hoses in the cabin. Almost surely your core is leaking.

steve, the FAQ says:

"Usually if the core goes bad you get a coolant mist on the windshield with the defrosters on , and you get some drainage from the A/C condensate line. The control valve may have spring a leak. Check that carefully, along with the hoses, before yanking the core out."

Are you sure there are no hoses?

Thanks.

edit: Never mind, just looked at the detailed procedure for the heater core, it says there are no hoses.
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Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'

Unfortunatly I think you already know what the problem is. I'm betting on the heater core being coroded and having a small leak or at a minimum the hoses have started to leak. The FAQ goes into good detailon what needs to be removed to get at it. To be sure you can put dye in the coolant and then see where its comming from with UV.

I think I'll do the dye suggestion just to make sure. Now to find a UV light and the dye somewhere. :-) Any suggestions?

Thanks!
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Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'

Can't see that dye will achieve anything as the leak is most likely the heater core which is hidden in a plastic housing up against the interior firewall. The source of the leak will not be revealed until you remove that housing.

FCP Groton have the heater core for around $100.

Removing the front seats makes the job less back breaking. A flexible shaft socket driver to remove the screws that hold the housing is the only special tool you will need.

Take care,
John H from Downunder