I had this problem about 3 years ago when I bought the car. (2000 A6 Avant). They ended up replacing the thermostat which is a timely job on these which didn't fix it. They finally ended up purging air out of the system (heater core) and it fixed the problem.

I now have the same problem with random cold air blowing out. Other times it will blow out hot/warm air. It seems to make sense that with the heat going in/out, there could be air in the system again. I took it to the mechanic I always use and he said he purged it but the problem is still there. What is the proper way to purge the air out, thought about trying it myself.

Thanks

 

Purge-bleed:

The system is self-bled - under one condition -> there are no internal flow-restrictions!

Heating on -> run the engine reaching the operating T->shutdown and leave overnight.
Level the coolant in the tank.

Do you observe any T-gauge movements coinciding with the cold air from the heater?

 

The temp gauge stays consistent when hot or cold air is blowing which lead me to believe I could rule out the new thermostat. I will do what you suggested, thanks for the reply.

 

The car should be on level surface.

 

welp, I ran the car to operating temp, let sit overnight and checked the coolant level on a level surface which was full. This has me baffled and might take it to our authorized Audi dealership. Maybe the cooling system needs a complete flush, pressure check and fill?

 

Sounds like your temperature controller is going on the blink. What happens when you put the heat all the up on the highest setting? Do you still have the problem? If not, it is your controller, not the coolant, and you are chasing down the wrong path.

 

that's where the problem is: when you have it set to the highest setting for heat, it will blow cold when at a stand still (car at normal op temp), to semi warm and sometimes hot when driving. It fluctuates when driving. I also noticed that if I turn it off for a couple minutes, then turn it back on, it'll blast hot air for 10 seconds then peter off to semi warm air coming out.

 

Curious, what happened with this?

 

Nothing much, the heat is still acting weird. I'm not sure where to go with it.

 

How much of a challenge are you willing to take on? Your next move is to prop open the heat mixer door so that you get constant air over the heater core. This will remove the heat controller from the loop. If you still get the weird action, you probably have a blocked heater core. If not, then you could replace the inside thermostat and see what that does for you.

But, I highly suspect that getting to the heat mixer door will be adventurous. Maybe someone else can chime in with a cheat for getting to the door.

Bob