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Old 02-08-2019, 11:58 PM   #1
PNW760
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Default Heat stops blowing warm after idling too long

Lately now, if my car sits longer than 30 seconds-1 min, it starts to blow cold air and heat seems to not work at all.

Also it seems to be that the fan turns on and sounds like itís going crazy any time it sits longer than 30secs to a minute.

Car doesnít overheat and no condensation that would normally occur from a bad heater core so Iím really not sure what it would be...
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Old 02-09-2019, 02:53 AM   #2
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You need a new thermostat.
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Old 02-09-2019, 10:15 AM   #3
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Either that or you are low on coolant
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Old 02-09-2019, 12:10 PM   #4
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Coolant slowly circulates through the heater core while it is idling, thus cool air can quickly cool it back. Either the heater valve cant open fully, or there is blockage or limescale in the heater circuit.

Whats up with the fan? Which fan? Heater fan or e-fan at the radiator?
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Old 02-09-2019, 12:26 PM   #5
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Do you still have that heater control valve under the hood that is plastic and likes to blow up? I'd ditch that.. It's right behind the head hooked into the heater hoses. Controlled by a vacuum line.

On my 91 740 has that one flapper that opens cold air from outside and it is electric and it was stuck open. I had to swap that for one that worked, you know closes the cold air from outside. Other then that vacuum operates the other flappers and that heater control valve maybe you got vacuum leak and stuff ain't working like it should at idle..
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Old 02-09-2019, 03:06 PM   #6
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You need a new thermostat.
Wouldnít a bad thermostat cause a temp drop in the in car temp gauge?
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Old 02-09-2019, 03:09 PM   #7
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Either that or you are low on coolant
I just checked and I am a little low, the car doesnít overheat though so I donít think itís quite that low. But would having low coolant simply be enough to do this?

Also if I do replace my thermostat, I might as well do a coolant flush while Iím there?
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Old 02-09-2019, 03:12 PM   #8
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Originally Posted by sbabbs View Post
Do you still have that heater control valve under the hood that is plastic and likes to blow up? I'd ditch that.. It's right behind the head hooked into the heater hoses. Controlled by a vacuum line.

On my 91 740 has that one flapper that opens cold air from outside and it is electric and it was stuck open. I had to swap that for one that worked, you know closes the cold air from outside. Other then that vacuum operates the other flappers and that heater control valve maybe you got vacuum leak and stuff ain't working like it should at idle..
Hey sbabbs, I still have the heater control valve, if I were to ditch it, how would I do that? Just loop the lines? Or is there a special procedure? Iíve heard these can blow up from time to time and considering the 940 has 324k miles, itís either a replaced unit or a ticking time bomb
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Old 02-09-2019, 03:59 PM   #9
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Take it out and one of the hoses and just attach the other hose that also went to valve to the firewall instead. Or put connector pipe where the valve was. That valve blew up on that 940 wagon I bought the other day, blew head gasket cause I kept driving..
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Old 02-09-2019, 06:28 PM   #10
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.
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Old 02-09-2019, 11:00 PM   #11
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Quote:
Originally Posted by PNW760 View Post
I just checked and I am a little low, the car doesnít overheat though so I donít think itís quite that low. But would having low coolant simply be enough to do this?

Also if I do replace my thermostat, I might as well do a coolant flush while Iím there?
On a 7/9 series if you don't have any coolant in the overflow bottle, you may be low enough on coolant that the heater core does not get a steady flow of coolant. You have to be about 1/2 gallon low to cause that problem.
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Old 02-11-2019, 05:49 PM   #12
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After topping off the overflow bottle with 50/50 water and Volvo coolant, the heat seems to have come back. I’ll update again in a few days just to make sure that was indeed the problem
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Old 02-14-2019, 03:20 PM   #13
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To fully confirm, my heat is now working perfectly. The electric Fan doesn’t kick on either, so it seems that being low on coolant is enough to cause your heat not to work. Thank you everyone for all the help!
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Old 02-14-2019, 03:31 PM   #14
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Cool. Or should I say, warm?
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Old 02-14-2019, 05:51 PM   #15
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A Parian Hilton “That’s Hot” will do
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