YKallroad

I followed the good advice of countless posters and hooked some tubing to the inlet and outlet ports of my heater core. Garden hose on one end, draining in to a bucket on the other.

A little coolant came out (probably .5L or less) then just clear water. Hmmm...where's all the gunk and crap that everyone's been finding?

Okay, step two. Emptied a half bottle of CLR into the heater core, diluted it with a bit of water. Washed the motorcycle quickly, then flushed the heater core.

Hmmm...still clear? Weird. In goes the second half of the bottle of CLR. Waited 10 minutes. Flushed. Same (clear) result.

The point is, I was assuming that my core was gummed up because when it's cold outside (below -15C) I don't get heat at idle, only at ~2000RPM or above. So now I am waiting until it gets cold again to see if (miraculously) this procedure (with no noticeable result) has fixed it.

Finally, so this isn't a completely wasted thread, here is one of the links I referenced:
https://www.audiworld.com/forums/a4-...w-pics-349252/

Jeff-AudiUSAParts

You probably researched this already, there are a number of electronic flap positioning motors on the heater box. 6 of them I count, but probably 1 of 2 that could be bad on your car. Fresh air flap and/or temp regulating flap of which there is a passengers and drivers side on that temp regulating flap motor. They all run 130.94 for the motor. I was hoping there was a heater valve but there isn't, since it's a flap motor system. I had an old Toyota Corolla that had this problem and it was a sticky heater valve. Back when systems were simple. Then again a car like that took forever to heat up the cabin too.

YKallroad

Can I troubleshoot that using VAG-COM?

Jeff-AudiUSAParts

I wish I knew more about the vagcom, but I bet it can help ID the issue. On newer cars those flap motors are reference in my system with a V, for example V107 might be for a temp flap motor and ID the exact part number for me to sell. On your car, I don't see any info to help figure out which is which other than what flap the motor controls. But, there is probably a part number on each motor so once in there if you see the part number then you've got it made. I guess what I'm trying to say is if the vagcom gives you a V number, in your case, get the part number off the flap motor to be sure. But, most people just tell us which flap isn't working and we can find the right one as etka does a good job of describing what the flap motor controls.

Old_School_Audi

Link to what I found on B5 heater cores

Now I haven't opened up a C5 heater core yet, but the symptoms you decribe could be an internal failure in the core baffle. The coolant takes the path of least resistance and bypasses the passages that allow heat transfer.

amiram

Did you drain air from the cooling system?
In the attached file, air release hole,on the entrance tube of the heater core

N'Syncro

Hmmm. I have a similar problem. Blows cold at idle at -20C or lower, but I also never get cool air blowing in the summer, unless the AC is on, so I was thinking air bubble and was going to perform a complete coolant flush pretty soon.

Kneale Brownson

Try bleeding the system by opening the bleeder screw (I think it's a 5mm allen head) in the hose in the middle of the engine Y near the front. Run engine with heater on, open screw till fluid runs out, close screw, repeat as needed (keep engine running/opening/closing screw) until you get heat at idle.

YKallroad

Thanks, I will try that. I haven't been able to find it, but I guess I wasn't looking in the right spot.

YKallroad

Thanks for the diagram. I did bleed it from the indicated hose (no air was trapped up there.)