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Cold Air Blows With Heater On Only At A Stop?!?!


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#1 rsafr

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Posted 24 January 2008 - 08:57 AM

I have an intermittent problem with my 93 LS400. The heat works great but occassionaly when I stop at a light or in traffic, cold air blows out the heater. Temp is at Max and it will go away after driving again after a few minutes. Anyone else run into this? Ideas?? Thanks

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#2 Leadfoot

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Posted 24 January 2008 - 09:11 AM

I have an intermittent problem with my 93 LS400. The heat works great but occassionaly when I stop at a light or in traffic, cold air blows out the heater. Temp is at Max and it will go away after driving again after a few minutes. Anyone else run into this? Ideas?? Thanks

Sounds like you might have a gas lock in the heater system, its the highest point in the cooling circuit and when you are idling the pump pressure is at its lowest. How is the fluid level in the coolant reservior?

#3 VMF

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Posted 24 January 2008 - 09:15 AM

i have the same problem on my 96, after research i found out that its a firmware bug. thats what they said about second generation of LS.

#4 rsafr

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Posted 24 January 2008 - 10:22 AM

I have an intermittent problem with my 93 LS400. The heat works great but occassionaly when I stop at a light or in traffic, cold air blows out the heater. Temp is at Max and it will go away after driving again after a few minutes. Anyone else run into this? Ideas?? Thanks

Sounds like you might have a gas lock in the heater system, its the highest point in the cooling circuit and when you are idling the pump pressure is at its lowest. How is the fluid level in the coolant reservior?

Fluid level is at the top / full level. I've been monitoring it. The only thought I have is that there is a "bubble" somewhere in the system. Typically, does the system need to be purged?

#5 scottmcc

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Posted 24 January 2008 - 10:47 AM

Are you saying that your temperature gauge goes to max and then returns back to normal operating range once you start driving again..?

I would check your thermostat first. Your engine can he hot while your thermostat will not open to allow flow into your heater core causing it to blow cooler air..

With Temperatures as cold as they are right now 0 degrees and below make sure your coolant is good for those temps as it can freeze and stick your thermostat...

#6 eatingupblacktop

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Posted 24 January 2008 - 11:10 PM

When you say "Temp is at Max and it will go away after driving again after a few minutes" are you refering to the climate control setting or your temp gauge? If you're getting normal heat when driving as you describe and no heat when idling, I would check your thermostat and test the coolant mix as scott has mentioned. If you've got old coolant and didn't use distilled water when last mixed, you might have deposit buildup partially blocking your thermostat or elsewhere in the system, including your heater core and pump. An air pocket is also a possibility.

#7 drrod

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Posted 10 February 2008 - 11:59 PM

My SO's 01 LS does the same thing. Only thing different is that when the transmission is put into P, it blows warm again. This just started so we haven't diagnosed the problem yet.
I posed the qestion on the forum just now. Should have done a search first and found this thread. Sorry.
Rod

#8 wwest

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Posted 11 February 2008 - 10:52 AM

I have an intermittent problem with my 93 LS400. The heat works great but occassionaly when I stop at a light or in traffic, cold air blows out the heater. Temp is at Max and it will go away after driving again after a few minutes. Anyone else run into this? Ideas?? Thanks


The OAT, Outside Air Temperature sensor, is located behind the front bumper just in front of the A/C condensor/radiator "stack". while stopped or moving slowly in heavy traffic it will often be heated via radiant heat from the condnesor radiator and that will sometimes result in coolish airflow from the system. Worse yet, it will sometimes premature "trigger" the climate control's automatic switch from heating mode, primarily footwell outflow, into cooling mode, footwell/dash combined outflow.

Brrrr....!!

Later models, mostly those that also use the OAT sensor for OAT indication to the driver, use a bit of digital signal processing so the HVAC system does not react to short term (false, temporary) temperature excursions.

I moved my '92's OAT sensor into the right front bumper area, well away from radiant effects of the condensor/radiator.

If you truly have the temperature setting at MAX, fully at MAX, then ignore all of the above and focus on a vapor lock in the coolant system.

With the temperature setting at MAX heat or MAX cool all sensor inputs are ignored.

#9 dtob

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Posted 05 January 2009 - 11:18 PM

I just found this post. I too have a 96 LS400 with the same cold air blowing issue and wondered if the original poster fixed their problem - was it an vapor lock or sensor issue. I'm going to test tomorrow by setting my temp to MAX. I think I've done that in the past though, so I may be looking at a vapor lock.

#10 astar617

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Posted 06 January 2009 - 03:48 PM

I just found this post. I too have a 96 LS400 with the same cold air blowing issue and wondered if the original poster fixed their problem - was it an vapor lock or sensor issue. I'm going to test tomorrow by setting my temp to MAX. I think I've done that in the past though, so I may be looking at a vapor lock.

There's a much more recently active thread which suggests checking the heater control valve for a stuck halfway open condition. That's what I'm checking on my 96 next, once I get time (and the New England cold subsides just a bit... btw, no way that sensor placement is playing a role when its 10F outside pre-windchill).

#11 PureDrifter

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Posted 06 January 2009 - 04:12 PM

i'd start by purging the air out of the cooling system. if problem persists, most likely thermostat and THEN heater valve.

#12 carlhpretired

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Posted 27 January 2009 - 02:07 PM

I have an intermittent problem with my 93 LS400. The heat works great but occassionaly when I stop at a light or in traffic, cold air blows out the heater. Temp is at Max and it will go away after driving again after a few minutes. Anyone else run into this? Ideas?? Thanks



I have a 1990 LS400 that developed the same problem - baffled me for months. It turned out to be a defective radiator.
These are aluminum radiators and have extremely small passages that are easily clogged if you inadvertently use or top off
with the wrong (i.e. non-Toyota) coolant. Some of these passages had become clogged in my radiator and it had to be replaced.
A new radiator solved the problem and is has never recurred.

#13 97flemingLS400

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Posted 28 January 2009 - 01:39 PM

I have an intermittent problem with my 93 LS400. The heat works great but occassionaly when I stop at a light or in traffic, cold air blows out the heater. Temp is at Max and it will go away after driving again after a few minutes. Anyone else run into this? Ideas?? Thanks



I have a 1990 LS400 that developed the same problem - baffled me for months. It turned out to be a defective radiator.
These are aluminum radiators and have extremely small passages that are easily clogged if you inadvertently use or top off
with the wrong (i.e. non-Toyota) coolant. Some of these passages had become clogged in my radiator and it had to be replaced.
A new radiator solved the problem and is has never recurred.


I have exactly this same heater hot/cold issue with my '97 LS400. Thanks for the suggestions -- I'll check things out and let you know what I find.

#14 Vandal

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Posted 07 February 2009 - 05:01 PM

I have an intermittent problem with my 93 LS400. The heat works great but occassionaly when I stop at a light or in traffic, cold air blows out the heater. Temp is at Max and it will go away after driving again after a few minutes. Anyone else run into this? Ideas?? Thanks



I have a 1990 LS400 that developed the same problem - baffled me for months. It turned out to be a defective radiator.
These are aluminum radiators and have extremely small passages that are easily clogged if you inadvertently use or top off
with the wrong (i.e. non-Toyota) coolant. Some of these passages had become clogged in my radiator and it had to be replaced.
A new radiator solved the problem and is has never recurred.


I have exactly this same heater hot/cold issue with my '97 LS400. Thanks for the suggestions -- I'll check things out and let you know what I find.


Here is the answer, most likely for all Lexus owners. I have a 1994 SC400. No heat. I found a web posting at http://en.allexperts...8/f_4590148.htm

Follow their directions. This fixed my heater. I think there is a fundamental flaw in the Lexus design. When the fluid level gets low you get a gas pocket that lives in the heater core. This keeps the fluid from flowing through the heater core. I loosened the 17 mm bolt at the top of the thermostat and ran the car at 2500 rpm. At the same time, have the coolant cap off and top off the coolant over and over until the coolant level is finally full. You will get heat after this. Retighten the 17 mm bolt, secure the coolant cap and be happy!

If this works, please pass it on. This will save a lot of Lexus owners a lot of grief and money. It took a total of five minutes to fix!! Yeah internet!!!!

#15 AzHotLS

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Posted 07 February 2009 - 06:14 PM

Vandal, good suggestion.

Except for removing the water inlet bolt, that's basically the instructions for purging air from the cooling system. There should be instructions on how to do it posted on a label on the reservoir. I added to it and came up with this:

1) Remove the tank cap
2) Set heater temp on high (85)
3) Let engine warm up to normal operating temp (to open the thermostat)
4) Rev engine between 2000 and 2500 RPM for 5 minutes (at some point you may see some fluid "blurp" (technical term) out as the air escapes)
5) Turn off engine and let engine cool
6) Add fluid to the tank as coolant is drawn back into radiator (don't let tank get empty)
7) When engine is totally cool, add fluid to full level
8) Replace cap

which I posted previously in this thread:

http://us.lexusowner...showtopic=53882

As for the inlet bolt, I'm told the 98-00 models don't have them.

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