DBJoe996

It almost sounds like your heater core is clogged. Remove right (passenger) side cowling next to the battery in the frunk. The intake and exit heater core hoses are right there. Run car up to temp and put your hands on the heater core hoses. If one is hot and the other cold, the core is clogged.

Nicolito

I will check the heater core tomorrow.

After reading a lot about, and trying to understand how heater works with the heat control flaps I more and more lean towards there somewhere is the problem, mostly based on that the heat sometimes work just fine which is strange.

I guess the cabin temp sensor should steer the heat control flaps open and close?
But when HVAC is on Defrost the temp sensor should be bypassed forcing system in full heat correct? So maybe not cabin temp sensor.

Leading me to electric problems in HVAC or the motor/servo for heat control flaps.

Still can't understand why when Defrost I don't even get cold air to front window or is that a direct flow not possible if heat control flap is closed?

I'll get back with further conclusions :-)
// Nic

dcdrechsel

The servo you were looking at controls the heat blend door .The heater (small radiator ) is always on -the blend door directs airflow around the heater and into the cabin .There are very few moving parts in this system so shouldn't be too hard to figure out what is wrong .The coolant hoses going to the heater behind the battery are marked in and out .They should be around the same temp -with the car warmed up if the outgoing hose is cold heater is plugged .Since you have intermittent heat -highly doubtful .There is another servo for the defrost flap ..Since this is intermittent you might want to check the cabin temperature sensor in the passenger side air vent it could be dirty ,loose connection or broke .II don't know what reference material you have but a porsche parts diagram might help you visualize how the system functions .

Nicolito

Today I ran the car until it was warm and I checked the IN and OUT hoses from the heater core and they where kind of equally warm, not burning hot but warm.

After that I pulled out the cabin temp sensor, disconnected and cleaned as good as I could. Mounted all back and started up the car and the heat is on!

Have used the car twice today in snowy Stockholm and it seems to be working just fine so I hope it continues that way now!

Thanks to everyone for tips!!!
// Nic

dcdrechsel

thanks for the update...........stay warm

Nicolito

Okay, problem back again!!

This morning when I started the car and went for a drive only cold air again. Drove the car warm for 30 minutes and parked - after approx 30 minutes I started again and now the heat came back and Defrost and all options on HVAC worked again.

So strange behaviour. Any clues?
// Nic

DBJoe996

Hate to say it but a lot of weird electrical gremlins are a result of the electrical part of the ignition switch. Sometimes it works and sometimes it wreaks havoc with various components. Might consider replacing it. There are some good tips, hints and videos on exactly how to do it. Search.

dcdrechsel

This might sound like a dumb question but when it produces heat is it really hot with the temperature setting at max ? Before I fixed my blend door I would get lukewarm heat after running at highway speeds for 20 minutes .That was in 40 f weather ...

Nicolito

DBJoe996: you mean changing ignition switch or the HVAC unit?

Nicolito

dcdrechsel: when it is working it is good heat in the system!

dcdrechsel

If the blend door isn't functioning or in the wrong position - no heat assuming the fan is working .Logical test would be to see where the blend door lever is at with heat and without .I would check at max heat temp setting .It would seem that there is a temperature sensitive connection somewhere in the circuit .One simple step could be to reseat the connectors on the climate control unit .

dcdrechsel

more .....the hvac system is all Audi components - the air handler had a Audi tag and part # -point being there may some local to you shops with Audi hvac test equipment .

DBJoe996

The electrical part of the ignition switch is what I mean.

golock911

Several times you've mentioned that after the car has been warmed up and then restarted, the heat is OK. Is this always the case? When the heat is working will it redirect between the windshield and the floor? It seems like something gets stuck when the car is cold and is held in place until you shut off the warm car for a moment. When you checked the heater hoses, was the heater working at the time? Can you recheck the hoses when the heater is working to see if the temperature is any different?

Nicolito

Hi
Yes, when I have driven for 20 minutes and car is warm and I shut it off for a while it almost always works after that.
I will check hoses temp when heat has been running in the car for a while to see if any difference in heat.
I'll get back!