Sorry, I should have posted this in summer!
I drive mostly long distances, and after a half hour or so, I regularly begin feeling hot in this car. It has always been hotter than inside others. Especially so after about half an hour's drive or longer, uncomfortably hot. Initially I thought it was the sun heating the black plastic dashboard, but I wanted to get to the bottom of this and try to do something about it, if anyhow possible.
The findings were astonishing, and may affect you too! Here is what happened and what was done to correct it:
I grabbed a digital thermometer with an external sensor. I put the sensor out of the window while driving on the highway, and measured 31°C outside temperature. With the heat all the way off, and the air blower at full blast, I put the sensor into the air stream blowing into the inside. It measured 3.5°C (6°F?) hotter than the outside air!
I had to recheck. The outside temperature was as before. This time I put the sensor into a different outlet blowing into the car, and it too was 3.5°C hotter than the outside air.
My next step was to check if the hot air flap of the heater assembly was closed all the way.
This is what a fully closed heater flap (heater all the way off) looks like: Observe the rearmost lever standing vertically up.
This is how the heat control flap looks like with the heat fully on: Observe the rearmost lever being horizontal.
If the hot air flap does not close all the way, the cord that pulls the flap can be easily adjusted to make the rearmost lever stand upwards and firmly close the flap. To adjust it, you could unclip the cord out of its holder and clip it back in correctly. See picture:
In my case the heater flap is correctly closed all the way, meaning the heater is turned off fully, but after time the car gets hotter and hotter because of warmer air blowing to the inside!
The reason for this can only be that the now hot heater assembly heats the "cool" fresh air going through it, albeit with the heat correctly turned off! It is simply a primitive construction!
What can be done about it?
Since I don't want to simmer and dehydrate in this uncomfortable car, I built a coolant bypass for the heater core. It is in place in summer, and gets taken back out in winter. I did not want to pinch off the heater hose in an effort to stop the coolant flow, as that flow has other important purposes and must not be interrupted.
What difference does it make?
With the bypass in place and under the otherwise same conditions as before, the air blowing inside was now practically the same temperature as outside.
It was now much better inside the car, the difference was almost as much as turning the airconditioning on! It is like having one airconditioning unit secretly and constantly on, without needing to turn on the real one!
I would be interested in knowing if other folks can measure the same temperature differences between the outside air and the air blowing to the inside, and that with both the heat and the airconditioning turned off. To measure the difference, the warmed-up car has to be driven at least a half an hour to get the heater assembly itself hot enough to heat the car without the heater turned on.
The following picture is my coolant bypass for the heater core. It is made of simple copper pipe, with an outside diameter of 18mm (medieval and non-standard: .71 inches). A local plumber could easily make it at little cost.
The bypass tube goes onto the rubber hoses connecting the heater core.
The following picture is without the tube connected, because the cold season has begun in this part of the world.