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Discussion Starter #1
I tried and searched for this issue in this forum and not sure if it is the new format here or whatever, I can't seem to find posts related to my problem. I always set the inside cabin temperature to 72 degrees for both left and right sides and use AUTO mode. Started recently, if I don't have my AC turned on, the middle level vents (the ones on the dash directly blowing to our face) and the foot level vents would be blowing very warm air in AUTO mode. I made sure the center rotary stratified air temp dial (the dial between the two center vents) is on the 3 blue dots (coldest) but it is still blowing very warm air to a point that it was unbearable that I have to open all the front windows or I have to turn the AC on even on a cool fall day just to get some cooled air blowing inside the cabin. Occasionally, the right side of the vents would blow cooler air as compared to the left side blowing warm air and sometimes vice versa. Whenever I turned on the AC, all is fine and blowing chilling cold air in all vents so I assume my AC is working fine. Any clue what might be the problem here ? Is this a typical symptom when the dual heater valve going south ? I think I see our heater valve has an auxiliary water pump attached to it while other E39 models do not seem to have it, is this correct ? Are the dual valves getting stuck mor the auxiliary water pump failing ?
 

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Do you get cold air blowing when moving, but warm air when at a stand still? If that is the case, your auxiliary fan might be dead. No air moving over your compressor when at a stand still, you will get warm air blowing through the vents.
 

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Discussion Starter #4
Do you get cold air blowing when moving, but warm air when at a stand still? If that is the case, your auxiliary fan might be dead. No air moving over your compressor when at a stand still, you will get warm air blowing through the vents.
I get warm air blowing either moving/driving or idling.
 

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It might be that the blend door to the outside is stuck open, allowing hot engine bay air into the system. Simply a guess. Try changing the HVAC settings including manually adjusting the outside air button. For physical inspection, you could set the HVAC to the door closed position (air recirc.) and then remove the cabin air boxes to see if the doors are closed.
 

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Discussion Starter #6
It might be that the blend door to the outside is stuck open, allowing hot engine bay air into the system. Simply a guess. Try changing the HVAC settings including manually adjusting the outside air button. For physical inspection, you could set the HVAC to the door closed position (air recirc.) and then remove the cabin air boxes to see if the doors are closed.
Thanks and I will certainly give that a try.
 

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OP - curious if you figured this out. My new to me m5 is exhibiting similar behavior. I can set the temp to 60 (lowest setting) and it will still blow warm air even if it's 70 outside. If i turn on AC, everything is fine. Where are the aforementioned blend doors located?
 

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Discussion Starter #8
I solved my problem when I opened the front hood and discovered the driver side curve shape black plastic air channel tube from the firewall that connects to the cabin microfilter housing was not connected. This was because I had an independent shop changed the brake fluid a few weeks earlier and the tech must have forgotten to reconnect it after securing the cabin filter housing. After I reconnected it, all is fine again with the HVAC functions normally inside the cabin.
 
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