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I noticed a new (to me) issue while driving my 1995 Impala in to work this morning. I have the temperature selector set to full hot, fan on high, function selector set to heat, vent, bi-level, etc. At idle, the air blows cold, but when the car is moving or when I bump up the RPMs while at idle the temperature comes back up. The heat is just fine while moving. I can think of two things that might change heater behavior based on RPMs: the water pump and engine vacuum. I did change out the climate control head unit a while back to fix a broken temperature selector switch; could there be a vacuum leak in the head until that produces these symptoms? The temperature guage only comes up slightly at idle and the car doesn't overheat, so I think the water pump is doing it's job. What could be going on here?
 

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Pretty sure there is one hose on the intake that you can pull to make climate controls stop working. if you try that and drive around and heat still works that same way i would think it's the WP or you have a clogged heater core that only works with high volume going through it.

i think hose is on PASS side of intake close to firewall.
I take it you have stock T-stat ?
-ALF out...
 

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Thanks for the reply! I don't think it's a clogged heater core. I flushed the cooling system not too long ago and had no trouble at all running water and air through the core. The core itself was replaced within the last few years, too. Yes, stock thermostat. I'll look into that hose.
 

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There is a check valve on the hose to the heater unit. I believe the temp control valve is mechanically operated on an Impala. You could have a collapsed restrictor that feeds the heater core.

You may have a 160 deg. thermostat, or your thermostat may be stuck open.
 

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I don't think it's a clogged heater core. I flushed the cooling system not too long ago and had no trouble at all running water and air through the core. The core itself was replaced within the last few years, too.
Do you have an infrared gun? They're getting more and more inexpensive as they get better and better.
You could use it to check the temp of the heatercore hoses at different spots between the H2Opump & the heatercore.
 

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Do you have an infrared gun? They're getting more and more inexpensive as they get better and better.
You could use it to check the temp of the heatercore hoses at different spots between the H2Opump & the heatercore.
The thermostat housing measures 162 degrees or so when hot, so I either have a 160-degree stat in there or a stat that's opening too soon. I'd rather run the stock 180-degree stat given my stock tune.
 

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Looks like you live in a climate where the fact that the lack of heat at idle is important. Good news is, once you start moving, your heat is fine. So …

Do you still have that variable restrictor thing that's supposed to protect the heatercore from overpressurization at high engine RpMs?
Maybe it has a small piece of gunk in it that's affecting it's ability to flow enough at idle?

Unlike 91 & 92 B-cars, there is no temp-dependent or driver-dependent control of the coolant between the H2Opump and the heatercore.
Only things that regulate coolant flow to the heatecore are the engine RpM, and that weird plastic variable flow restrictor thing.
AIRflow through the heatercore should be regulated by the temp control unit. Maybe, at idle (OEM idle gets as low as 537.5RpM @ or over 185°F / 85°C), something is coming loose?
Although I totally appreciate why you'd want to go back to the OEM 180°F thermostat, the jury is still out on whether or not the OEM 180°F tstat is NEEDED for adequate winter heating.
I know of at least one person in Staten Island who's heating works almost too well on windy 20°F winter days, whose ImpalaSS has a 160°F tstat (we could not take sustained max heat).

IFF you've a 160°F thermostat for the winter … do you have good heat?

I, on the other hand, have an OEM 180°F tstat, and INadequate heating during any headwind; standing still with the wind at my back, heating can approach uncomfortable as it should.
(primary suspect: heatercore foam seal, 2ndary suspect, subpar sealing of the temp blend door, no other suspects yet).
This paragraph should not be construed as a cry for help, which would be a threadjack here - I only offer my experiences and testing for your consideration in the hope that it helps …
 

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Discussion Starter #8
Yes, I still have the OEM-style restrictor in place. It was clear when I flushed the cooling system a month ago. The hoses going into the core are definitely hot (input) and warm (output), so I have more investigative work to do.
 

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There is a check valve on the hose to the heater unit. I believe the temp control valve is mechanically operated on an Impala. You could have a collapsed restrictor that feeds the heater core.

You may have a 160 deg. thermostat, or your thermostat may be stuck open.
 

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On my '94 Impala I had the same problem. I took a garden hose an hoked it to the heater core and flushed it out (both directions) that helped. Then latter I discovered a heater hose that had a twist/kink in it. That was my problem.
 

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SAH62,
In your case, I'm wondering how the temp blend door operates 'cause I don't know, 'cause I'm ignorant here:
entirely mechanically - if the engine has been off for a while, does the temp control knob still swing AND SEAL the door completely independent of vacuum?
or
does engine vacuum play an important part in the proper sealing of the temp blend door?

Almost sure that the door that directs air from 'Prince' (windshield defrost), to 'Blend', to 'Heat' (feet), to 'Vent' (torso), to 'Bi-Lev', to 'Max' (recirc torso), uses vacuum.
(Not my best post, just thinking aloud …)
 

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Disconnect the hoses to the heater core (plug the "T" with a cork and turn the other hose upward and wedge behind the Alt. so they don't leak out. Back flush your heater core using a garden hose and a nozzle that will fit in the hose end. back flush with good pressure. put it all back together and bleed the air from your system.
 
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