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Fan air hotter than atmosphere?

2013 Mazda 5

Driving on Freeway in Califor around 3:30pm - drove 30-40 mins around 70mph without AC/Heater.

All of a sudden traffic slowed it down to 5-20mph. I noticed the fan air being blown was hotter - so when the window was down I noticed outside is cooler! Why was this? I also noticed some oil burning smell briefly - this would have been from the last power steering fluid flush spill - right after the change, I noticed oil on the splash cover too.

I happened to have my OBDII device connected for the first time. Smartphone app showed boiling temp. I drove back around 10:30pm - no issues.

I had driven 5-7hrs on freeways and have never noticed anything like this - but those drives would have had the AC turned on.

Does a hot engine can cause the fan air being blown to be hot for the passengers?

Is this a normal?
In my 20 or so years of driving, this is the first time I experienced. Does this have anything to do with the speed around 10mph - I guess more cooling when the car runs faster?

Turn the Ac back on . Problem solved .

Fresh air for the HVAC system is drawn in though the cowl in front of the windshield. At slow speeds hot air escaping from the engine compartment may be drawn in.


Many cars have engine coolant circulating through the heater core all the time. You may feel this as radiant heat. I found this uncomfortable and have preferred cars with a valve that can close off coolant flow to the heater core.

Flaps and doors in the HVAC ducts open and close to control the amount and direction of air flow. Maybe there’s a bit of leakage, so warm air is coming out of some ducts when it is not desired.


Are cars with the valve controlling the coolant flow made anymore? I haven’t seen one in 20 years.


Dunno. 20 years may be about right: my 1999 Honda has a valve on the firewall that regulates coolant flow to the heater core as the HVAC Hot-to-Cold dial is turned. That’s one reason I bought the car. My prior car, a 1984 Chevy Cavalier, had constant flow to the heater core, and I disliked the radiant heat at my feet.

It is quite possible that the heat and cooling systems have been improved since 1984 .

Valve? Most likely no :slight_smile:

My 2006 Pathfinder has an electric pump pushing coolant via heater core, so I suspect circulation is quite restricted by it, until HVAC calls for heat.

Some have a valve that shuts flow to the heater core when max a/c is selected. My '04 and '08 Chrysler Pacificas did.