Blend door/flushing heater core/t-stat

I was wondering I keep hearing about a “blend door” when it comes to heating/cooling a car. I know it’s located in the dash on the passanger side, but what is it and how does it work and effect your car’s temp?

Also, if you had to have your radiator replaced ( I did) would all the fluid be drained out including the fluid in the heater core?

What signs if any can you look for to see that your thermostat is stuck open? I figure if it was stuck closed your engine would overheat, but what about stuck open?

the engine takes longer and sometimes depending on the driving distance and temp outside, it will not reach operating temperature, so the car will run in open loop most of the time, wasting gas and not running properly. Also heat would not work right. The blend door gives you more or less heat in the cabin, but would not have a effect on performance unless it is vacuum controlled and it is leaking. When you replace the radiator, I recommend a flush depending on the reason you had to change it in the first place. If I drain the fluid and it is rusty and full of debris, I will have to use chemicals to flush the cooling system. You have to keep an eye on which coolant you are using and respect change interval. Do you drive a Dodge?

My car is a 98 Ford Taurus SE stationwagon. My car reaches it’s running temp pretty much in a timely manner and I have seen the temp gauge get to point X and then drop down which I figure is the thermostat opening. I just had to have a new radiator put in my car about 2 weeks ago, so wasn’t sure if when you do that if they have to, or should also remove all fluid from heater core. As for my blend door I am getting air out but it is cold and never gets warm.

Your blend-air temperature control door is controlled by an electric motor, not vacuum. This is true if you have manual or automatic climate control. The air flow direction doors are vacuum controlled.

On your AC system, all of the air blown into the cabin always passes through the AC evaporator. The blend-air door controls how much of that air then passes through the heater core, and how much bypasses the heater core. For full cooling, it all bypasses the heater core. For full heating, it all goes through the heater core. (It still goes through the AC evaporator either way.)

My luck my door is positioned just for cool air. And from what I have heard, getting tot he blend door is a PITA to get to…so we will have to try and warm the car a different way.

Google for the problem. Some people have come up with some clever workarounds that do not require major dis-assembly.

Note that the drop in temp that you see is more likely caused by the radiator fan turning on rather than the t-stat opening.