Heat comes on strong then becomes cold

steveyblack

steveyblack

Member
Jun 21, 2007
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well this problem started today...When i turn on my heat it blows nice and hot then it quickly becomes cold...now if i leave it off for a few minutes and turn it on again after a few minutes, it will do the same things

I just replaced the thermostat wens., and the heat was working fine till today...

any one have a direction where i should look?
 
W

Wayne G.

87Turbo 5sp Targa
Apr 19, 2005
213
0
0
Gainesville,Ga.
steveyblack;1181265 said:
well this problem started today...When i turn on my heat it blows nice and hot then it quickly becomes cold...now if i leave it off for a few minutes and turn it on again after a few minutes, it will do the same things

I just replaced the thermostat wens., and the heat was working fine till today...

any one have a direction where i should look?
Click to expand...

Have you checked your VCV (vaccum control valve) for the heater. Its under the wiper motor on the firewall. Mine worked intermitant like that before failing completly.
 
steveyblack

steveyblack

Member
Jun 21, 2007
529
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Wayne G.;1181275 said:
Have you checked your VCV (vaccum control valve) for the heater. Its under the wiper motor on the firewall. Mine worked intermitant like that before failing completly.
Click to expand...
No, its dark and cold outside..ill check tomorrow...how will i know if it is working or not
 
89turbotoy

89turbotoy

2 89 supras 2jzgte/single
Jul 30, 2006
309
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cincinnati
just by pass it. take the vacuum hose and go straight to the top of the valve, if you have heat then just leave it that way.
 
Last edited:
W

Wayne G.

87Turbo 5sp Targa
Apr 19, 2005
213
0
0
Gainesville,Ga.
steveyblack;1181284 said:
No, its dark and cold outside..ill check tomorrow...how will i know if it is working or not
Click to expand...

Turn on the heater with the engine warm check the lever - see if it is all the
way up. When vacuum is applied the lever is pulled up and opens the water
valve letting water flow to the the heater. If it does not move up you can find a way to hold it open or route the vacuum line straight to the vacuum valve
with an extra piece of hose by passing the VSV and it will stay open all the time as a temp. fix.
Some of these guys bypass the whole valve setup and just run the water line straight to the heater----
 
steveyblack

steveyblack

Member
Jun 21, 2007
529
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Nj
sorry for not replying. I bhg'ed a couple days after and didnt use the car till recently(after a 2jzgte swap). So I never really figured out whats wrong.
 
I

IJ.

Grumpy Old Man
Mar 30, 2005
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I come from a land down under
The blend door is just that it's a mixer in the HVAC unit that swaps between Hot/Cold, you can totally remove the Heater Tap on the firewall and if the Blend door isn't working correctly you get no heat inside the car (or a small burst then nothing)

There is an amplifier in the Climate Control that drives it, I've had a couple of dead ones over the years, also there are mechanical linkages from the Servo Motors that move the door, they can come adrift.

You should "hear" the door cycle when moving the control from H to C and back.
 
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