I have an 01 S4 that we have been trying to diagnose heating issue for and not sure where to go now. At times, the heat works ok even at idle. Other times it does not heat the cabin at all, even if rev'd to 4k. It seems there is just no flow to the heater core at some times but we cannot determine why. Here is what we have done.

We have done the flush of the heater core, hooking up a garden hose and back flushing it. Water flowed nicely without any restriction and seemed to be clean.

We have bled the heater core to make sure there was no air in it. We have done this both by lifting the bottle and bringing the vehicle up to 4K and exposing the bleed hole, and also by pressurizing with the Audi unit that screws on to the cap on the coolant bottle. We get a solid stream of coolant out so there appears to be no air in the system. without the coolant being pressurized or the bottle being lifted though, there is no flow to the heater core.

We went through the climate control system trouble codes. The system shows 0.0 and is not giving any codes. Temperatures of the sensors all appear normal.

The engine is cooling just fine though. Leaving it at 5K RPM the temperature stays exactly where it is supposed to be. The temperature doesn't increase from what it does at idle. That would lead me to believe the new water pump is working properly.

Does anyone have any other ideas on what to check? I would think if at 4K rpm if we pulled the feed hose to the heater core off, it should just pour out coolant all over. However, we can pull it off at 4K and nothing more than a tiny trickle even comes out. Yet the hoses to the radiator seem to be rock hard and very well pressurized. It seems somehow the coolant flow to the heater core is being diverted and not even going that direction.

Any help is greatly appreciated.

 

i've had this same issue before. I flushed it with CLR and got a whole bunch of crap out of the heater core. This cured my issue for about a month. I then changed my axillary water pump. This did nothing, but was told it could be a cause. Finally i just replaced the heater core and everything has been good.

I'd say make sure you bled it correctly and check the aux water pump.

i used this method to bleed the system:

Slide the LEFT hand hose (looking from the front of the car) going through the firewall off the connection just enough to expose the tiny hole in the hose to air. Unscrew your overflow tank, disconnect the sensor, remove the cap and turn the car on. While holding the tank as high as you can, loosen the bleeder screw until the bubbles find their way out of it and you get a solid flow and tighten. Then, give the car a couple of GENTLE revs while you have the heat on high, let it idle and repeat as necessary until you get a solid flow from the heater core outlet hose.

 

The Aux coolant pump was replaced along with thermostat and water pump. I believe the aux pump only cools the turbos though doesn't it?

We have pulled the left hand hose off to expose the hole. We have run the car to 4K for awhile and at some times we get air/coolant sputtering out of the hole then a solid stream of coolant. At other times we do not get anything coming out at all no matter how long or hard the engine is rev'd up. If we use the Audi tool to pressurize the overflow bottle we can get a solid stream out of the hole and shove the hose back on. Once the pressure is removed from the bottle, sometimes we get some heat. other times we don't. We did this about a week ago. I had heat for a few days, then no heat for a few days. Now I have heat again. It has never reached the level of heat it used to though.

You mention a bleeder screw though. I have not seen that mentioned anywhere else in the guides for flushing/purging the heater core. Where is that located?

It almost seems like something is shutting off the flow to the heater core completely. There isn't even any flow coming out of the feed hose.
Does anyone have a diagram of where the coolant flow is supposed to go on the S4?