Windshield fogging and heat flowing at idle - system OFF - Chevy Impala SS Forum
 
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post #1 of 7 (permalink) Old 08-25-2016, 08:05 AM Thread Starter
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Windshield fogging and heat flowing at idle - system OFF

I think this is vacuum related, but asking here for confirmation.

With the windows down and the system completely off, upon startup or at idle the windshield will start to fog. You can feel heat coming out of the defogger vent, as well as a little coming out of the floor vents. I checked the vacuum port on the intake and its firmly attached and the lines are not broken, but I think the vac is leaking out somewhere causing this condition. Especially since it only happens at idle. Putting the defroster on will immediately clear it out, and the selector knob appears to function just fine when you actually select some other HVAC control condition. (Heat goes to the floor, max goes to the dash, defroster goes to the windshield, etc...)

Any ideas?


1995 Caprice ERE 383, CIA long tubes, Dynomax VT's, RAISS, Kore3/Z06 brakes, 3.42 posi, Boss 338 20's, 12 way seats, Impala SS console, etc etc etc

Last edited by atlantadan; 08-25-2016 at 11:33 AM.
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post #2 of 7 (permalink) Old 08-25-2016, 10:55 AM
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Sounds like a couple of issues.
Fogging? that shouldn't happen. Sounds like start of heater core going.
Feeling heat creeping out may indicate the temp blend door is not getting to the full cool position. Is it vacuum related?? idle is not your lowest vacuum. cruising under moderate load, WOT, accel, all of these will have even lower vacuum.
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post #3 of 7 (permalink) Old 08-25-2016, 11:36 AM Thread Starter
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Sounds like a couple of issues.
Fogging? that shouldn't happen. Sounds like start of heater core going.
Feeling heat creeping out may indicate the temp blend door is not getting to the full cool position. Is it vacuum related?? idle is not your lowest vacuum. cruising under moderate load, WOT, accel, all of these will have even lower vacuum.
Sorry, yes - you're correct about vacuum at idle. That's when its highest, actually. The heater core isn't leaking, heat works fine and there are no puddles or wet spots on carpet. Not losing any coolant, nor smelling any.

It fogs the windshield because the heat is blowing onto the cool glass. I can feel it coming out the defroster vent as well as trickling out the floor vent.


1995 Caprice ERE 383, CIA long tubes, Dynomax VT's, RAISS, Kore3/Z06 brakes, 3.42 posi, Boss 338 20's, 12 way seats, Impala SS console, etc etc etc
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post #4 of 7 (permalink) Old 08-25-2016, 12:15 PM
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Sorry, yes - you're correct about vacuum at idle. That's when its highest, actually. The heater core isn't leaking, heat works fine and there are no puddles or wet spots on carpet. Not losing any coolant, nor smelling any.

It fogs the windshield because the heat is blowing onto the cool glass. I can feel it coming out the defroster vent as well as trickling out the floor vent.
my guess is the heater core is leaking. this is how it begins. vehicle sits a few hours coolant system has pressure the coolant then leaks out in the air box some may drain out the drain tube but the heater core coils maybe wet . you start the vehicle and the blower blows slightly this then blows the moist air trapped in the box inside the vehicle causing the fog.

using a 18 psi coolant cap does damage ... if you have the 18 switch to the OEM 15 psi like my 96 came with back in nov 1995.
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post #5 of 7 (permalink) Old 08-25-2016, 12:19 PM
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vacuum lines doe not leak out they suck in .

with the vehicle off for like 30 min turn the hvac control knobs listen for the sucking sound if so no leak.
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post #6 of 7 (permalink) Old 08-25-2016, 02:52 PM Thread Starter
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my guess is the heater core is leaking. this is how it begins. vehicle sits a few hours coolant system has pressure the coolant then leaks out in the air box some may drain out the drain tube but the heater core coils maybe wet . you start the vehicle and the blower blows slightly this then blows the moist air trapped in the box inside the vehicle causing the fog. using a 18 psi coolant cap does damage ... if you have the 18 switch to the OEM 15 psi like my 96 came with back in nov 1995.
I get what you are saying, but there is HEAT blowing out the defroster vent, even when the system is 'off'. The air coming out is hotter than the ambient 85 degree day. The heater core is not leaking.


1995 Caprice ERE 383, CIA long tubes, Dynomax VT's, RAISS, Kore3/Z06 brakes, 3.42 posi, Boss 338 20's, 12 way seats, Impala SS console, etc etc etc
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post #7 of 7 (permalink) Old 08-25-2016, 09:53 PM
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The heat blend door is probably not working correctly, as well as the door that directs the air to the bottom and defrost vent. There are often strange things in the duct. The only way you can access the duct is by removing the drip pan. You can check for proper travel from the outside by watching the vacuum motors, but you will need a reference car. The vacuum motors themselves may be acting up, but I do not know the travel specification for them.


The tan hose goes to the lower side of the mode actuator, and the red hose goes to the upper side of the mode actuator. Left end of the duct, to the right of the defroster valve motor.


The blue hose goes to the defroster valve actuator. Left end of the duct.


The gray hose goes to the heater shutoff actuator. The valve should be behind the ashtray.

1991 OCC 461 (.030 over 454) BBC, 3.23 posi, flash to pass, drop spindles & springs, Impala rims, Hydroboost, Recaros, MOMO/wood SW w/QR, custom wood shift knob, Pioneer DEH P77DH
1992 OCC now with 5.7 tbi, DEH P77DH
For a parts list, check https://www.impalassforum.com/vBulle...ion-parts.html
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