1986 745 gle n/a 200k some miles, m46
I suffered a catastrophic loss of coolant today due to a broke hose by the heater core. The hose goes from the back of the engine to the firewall. There are 2 of those hoses. One of them I believe goes straight from the firewall to the block, the second one is split in 2 parts where in the middle is a valve of some sort. It is my guess that this valve controls the flow of coolant to the heater core so that I can control the level of heat. I can't find the name of this plastic valve in my Haynes. What is it called, and am I correct in what it is supposed to do?

Also, as a temporary fix I got a hose from autozone and just bypassed the valve. This is ok, right? I figure it should just enable maximum heating.

Finally, I replaced the all the hoses last time a radiator hose broke. How was I supposed to forsee this breaking?? Oh well.

Thanks,
Nathan

Hi Nathan, welcome to the exploding heater valve club!! Thankfully you had the wherewithall to shut your engine off. When it happened to my wife, she just kept on driving!!! The engine siezed, of course. That's when it became my car. I unsiezed it and drove for another three months with a leaking head gasket. The car now sports an almost new factory reman engine that I found in a wreck at the junkyard.
I've found that even the heater valves with metal vacuum chambers (FCP Groton's) still have plastic where the coolant goes through. These are definately a time rated item along with the hoses.

bagle 87 745T

The part is in fact the heater control valve. The new ones sold by FCP Groton are all-metal and won't fail like the originals. This is a really common 740 problem. Every single car out there should have a new valve in it by now. It's standard practice to replace it if the car has had any other cooling system failure, because it will be next to go.
As Kenric noted, it isn't expensive. And you are correct, that you can go a while without it, you simply have the heat on full if you put a piece of open pipe in there. Good thing you were paying attention to that needle- you saved yourself several hundred dollars worth of head gasket work.
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Rob Bareiss, New London CT ::: 86 244DL, 87 244DL, 88 744GLE, 91 244: 808K total

...with that upcoming heater core job, i might as well get a new valve too. probably going to order new stuff from fcp tomorrow.
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Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'

You don't need it to run your car. I had two of them break within a three month period. I also have the plastic one. I just bypassed it just as you did and I don't have any problems.

The valve only shuts all the coolent out of the heater core when you move the tempature control all the way to cold or if you turn on the A/C. If you move the control switch just about a cm from the coldest part its doesn't make the constant vacuum sucking noise as the car tries to activate a valve that it not there.
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-Brandon at FSU '88 744ti 150k miles

Oh, this makes lots of sense because today I noticed the heat worked fine without that valve. What does the slider inside the car activate that causes warmer or cooler air to flow?
-Nathan

It merely allow more or less amounts of air to pass by the heater core on its way into your car. The idea is that if you restrict all of the coolant from reaching the heater core there is no way of it heating your car by even a degree. Thus the reason for why the heater control valve only comes on with the a/c or full cold.
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-Brandon at FSU '88 744ti 156k miles

it happens... did your car overheat? i certainly hope not.

someone should get back to you for certain to confirm, but that's probably your heater control valve.

http://www.fcpgroton.com/volvo700900heating.htm#heater

$14

i have to get my heater core soon. be happy your core didn't break. :)
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Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'

[Did the car overheat?]
Nope, temperature needle got to just before the red area when I noticed it, and I swear I could see it moving upwards because I was trying to see if I needed to immediately shut it off... when I thought I saw it moving I shut it off within seconds. It's always a shock to see the temperature anywhere but 12 o'clock. This has happened once before with a radiator hose.

Can I get a heater control valve without the plastic? I'd be willing to pay 2x for that :)

Thanks,
Nathan

hmm... i dunno, i don't even know what the valve looks like.



is that it?

or this?



or this?


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

They are all heater valves that work as replacements for the one you blew. The top and bottom one are essentially the same thing... it looks like one is made by 4seasons and another by another company. The middle one is teh Bosch version, all plastic and generally avoided by most folks because of it (although its definitely the most attractive of the lot). The non-bosch are also a lot cheaper.

My opinion is that these valves just fail eventually, and you should consider it a wear item. Usually the valve will start to leak before it fails outright, so just pay attention and you should spot it. Open the hood right after a drive and listen for a hiss from the valve.

good luck,
rt

The first picture looks like mine best I can tell. Another poster said the fcpgroton.com valves are all metal and there is only one kind for all models so I should be fine.