Audi Repair Questions? Ask a Mechanic for Answers ASAP
The best way to bleed the heater core (since it sits higher than the reservoir opening) is to either drive the car up on a ramp slightly, or just run it to temp first. Loosen the clamp on the heater hose that has the weep port, but do not pull the hose back yet. Warm it up to temp, and while running, slightly slide or angle the hose until coolant streams out. You want it at temp so it will have good pressure. There should be air built up there as that is where it accumulates. So it may spit and sputter, but as soon as you get an actual "stream" of coolant, close the hose again. You'll need to let the car cool down again overnght, and then warm it up and do it again to see if you get any more air.
Just note that since the heater core is low, it will tend to attract "gunk". I have taken both hoses off and alternated with bursts of water and bursts of pressureized air (using a long flexible plastic wand to get the air down into the bottom is best) and you will see particles of rust flake, etc. I try to put a rag over it as best possible each time I blast with the air. Eventually when you don't feel the particles hitting your rag anymore flush through with clean water, put one hose on, fill it with water, and put the other hose on, this will help it get flow through with less air to purge
OK- as long as the flow switches from bubbles or air, to constant coolant flow, that's what you want. Having the system closed so it builds up a lot of pressure as it heats up is best, ***** ***** you open that port it really spits all the air out. If you can incline the car at all, say by driving it up on a set of car ramps in the front, that would help by lowering the heater core relative to the coolant reservoir.
When the car is up to temp, feel the hoses running into the heater core in the engine bay, make sure they are hot. It's always possible you have a bad coolant pump, this era was notorious for the impeller cracking and slipping on the shaft.
what engine do you have in this?
There is a secondary water pump, often coolant will migrate into the electrical connector and knock it out. I'm wondering if it is impeding your flow.
Sorry, busy weekend!
The secondary coolant pump is accessible from underneath the car- it is behind the alternator, just follow the coolant hoses to it.
It should run for sure after you shut the car down hot, as part of it's function is as an after-run pump to help cool the car down without hot spots.
Unless you have some other kind of obstruction in the system preventing flow, I can't see what else it could be. If you flushed out the core, don't see any gunk in the system, and are able to purge the air out of the bleed port until fluid runs out, then the only other thing that could be is some kind of obstruction in the lines. You said the hoses to the heater core were not getting hot.
I never addressed the possibility that your temp flap may not be making a full swee, because it seemed that the hoses were not getting up to full temp. NOrmal operating temperature is 175-195 for the engine, and that should come through the heater core at the same temperature. So I have been going on the assumption that you are not getting full flow in the system, for whatever reason.
As far as I know, the after-run pump is just that- activated only by the system to run after the fact- but I could be wrong, for whatever reason this is not clear in any of the literature. If you had a VAG-COM software or scan tool, you could run output diagnostics to see if it runs at all. The only way to be sure is to be able to feel or hear it run.