Weird heater problem
 

My car is blowing out only cold air when I turn the heat on. When I start driving the air gets hot and back to normal. As soon as I get to a red light and stop the car the heater would start blowing out cold air again. Why is this happening?

O5 RX8
Engine recently replaced
108,000kms
BHR coils

How's your coolant level? It sounds like hot coolant isn't making it to your heater core when standing still, but gets sloshed toward the back when you accelerate. Could be other things, but this should be easy to check.

Coolant level wouldn't make a difference. The heater core gets it's hot water directly from the back of the engine and lets it out directly to the water pump inlet. It's completely submerged and never connects to the bottle.

You should figure out if it's an RPM related change or a speed related change. If it's RPM related then that means that it's not flowing enough through the core at idle speeds (very low flow through the entire system), which would imply a clog somewhere in the heater core circuit.

That's why I was worried :uhh:

It COULD be an issue with the temperature control board - which is a very common problem RX8s have

It doesn't totally fit your symptoms, but if it only *seems* to be blowing hot when your moving and cold when you're not, it could be that.

The biggest symptom of that, is that the car blows out whatever it wants (but usually cold air on mine) no matter where you turn the dial. If it's not directly related to the RPM issue noted earlier, look into that.

If it is that, there are lots of DIY threads about fixing it. If you have even the slightest knowledge of how to use a soldering iron, it is a very simple fix.

I did the DIY guide and soldered my HVAC circuit board. It's worked great ever since :)