Heat stops blowing warm after idling too long

[PNW760]

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 it’s going crazy any time it sits longer than 30secs to a minute.

Car doesn’t overheat and no condensation that would normally occur from a bad heater core so I’m really not sure what it would be...

[Shifted]

You need a new thermostat.

[Mbeas96]

Either that or you are low on coolant

[petiww]

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?

[sbabbs]

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..

[PNW760]

Originally Posted by Shifted

You need a new thermostat.

Wouldn’t a bad thermostat cause a temp drop in the in car temp gauge?

[PNW760]

Originally Posted by Mbeas96

Either that or you are low on coolant

I just checked and I am a little low, the car doesn’t overheat though so I don’t think it’s 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 I’m there?

[PNW760]

Originally Posted by sbabbs

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? I’ve heard these can blow up from time to time and considering the 940 has 324k miles, it’s either a replaced unit or a ticking time bomb

[sbabbs]

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..

[alschnertz]

.

[2manyturbos]

Originally Posted by PNW760

I just checked and I am a little low, the car doesn’t overheat though so I don’t think it’s 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 I’m 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.

[PNW760]

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

[PNW760]

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!

[2manyturbos]

Cool. Or should I say, warm?

[PNW760]

A Parian Hilton “That’s Hot” will do