awdinut

My heater system will direct the air where I select, but the air barely gets warm even when set on "HI". That made me suspect a failure of a blend door motor or the like, but VAG COM says no faults in heat/ac system. And the temp gauge is right in the middle where it should be by the way. Also checked for air in system and there was none. Then had tech run the 'learn' program with the VAG COM to reset to factory settings...I think that runs all the doors etc.

Any thoughts about where to look next or what to try ? I went to heat/ac set up and made sure the center vent temp setting is on warmest. Nothing helps.

MP4.2+6.0

1. Run the VAG COM test in the HVAC module that cycles all the doors and various other things. See if anything awry.

2. Maybe you need to bleed cooling system?

3. After these, possible blockage/failure of heater core, or failure of the in line heater valve(s).
a. There is a TSB for older D3's that talks about premature core failure. Fix is...a new core. Patch is a cartridge kludge to get silicates back into the coolant. Better fix is to get off of old G12 and over to the later blends that brought silicates back. But that won't fix blockage if it's the core.
b. If like my W12 and I understand the system set up correctly, the heater valve that actually regulates hot coolant flow (an often forgotten part still in the system; it's not just a blend door) should be under the rear engine compartment cowl area to the (U.S.) passenger side of the HVAC filter. You should see two solenoids on the top of it, with hoses and wiring connectors going to it. A big bucks part, at least for a W12 (>$1,000). Maybe feel around for at least hot coolant. Each side of the front is separately regulated, hence I think the two electrical connections and solenoids. In a W12 there is more complication with an after run pump and some check valves, but I think that may not apply to a 4.2.

awdinut

Thanks for the reply ! I forgot to include in my post that we bled the system but got no air. Also the tech ran the 'learn' program that reset everything...is that the 'run' progrtam you mentioned. Also tech checked under pass side cowl cover and I guess the lines that should be hot were, at least he did not mention anything under there providing a clue.

Wow, a heater core...lets hope not !! There are two heater cores I think, and wouldnt both have to have failed at the same time ? Could be the car has always had whatever problem it has since I bought it with 60K and I never knew 'cause its never been this cold before and the system could keep up with teens and twentys temp.

MP4.2+6.0

I'm referring to the Output test functions of VCDS--the ones that cycle through a module and test out stuff. You will hear doors moving, fan spin up, etc.

Yes, two cores if I understand the same. But in turn that makes me suspect the valve block even more. Hoses need to be hot both before AND after the relevant solenoids. Might be worth pulling out to look at, or maybe bypassing temporarily with some connectors and clamps if possible to see if you then get good heater flow. I actually suspect the opposite on mine--it may be hanging open on one side when using HVAC, since in late summer I was getting hotter on one side than the other even w/ temps set identically on both sides.

I forgot to also suggest interior AND exterior temp sensors as another check area. I haven't really run down a lot, especially ones located inside. A couple of us (via Quattroworld info) have found changing the outside one seemed to improve AC function generally--in summer obviously--even though center dash display seemed correct in general. Not really sure why, and no idea what it might help if anything on heating side. It was cheap; only real issue is getting at it sandwiched between radiator and grill in a hidden area--at least on mine w/ fish mouth style. I managed to get at it though w/ some prying w/out pulling front clip area. May be easier pre fish mouth. In your case though, I would look at interior ones as more obvious. There must be a feedback loop to the blend doors telling them (and the water valve block and fan) to heat it up more (or not). IIRC one is maybe in the HVAC control panel behind the little slits, and another one is overhead (??) in the console. Look at Bentley and VCDS. I think there may be some kind of test for these too where you read measuring blocks. If those don't line up to the interior actual temp (as in, reading way too high), that would obviously throw the whole system off.

FWIW, maybe search for posts last winter/year by Volvopentaman (or something like that name)--he seemed to go crazy trying to find a basic heat problem in deep winter Scandinavia w/ a W12. I don't think he found it (at least while still posting), but it would be a set of ideas of what all he did as a guy w/ lots of apparent experience.

ltooz_a6_a8_q7

I think it's the heat/air is not flipped to heat. I had the same problems with my A6. How can the core is blocked when coolant is a sealed system? I don't know if you can find the flap or the system is different, but I doubt it.
http://www.dognmonkey.com/audi/no-he...udi-avant.html

Good luck,

Louis

MP4.2+6.0

Cores and radiators can get blocked w/ sediments and deposits. Drop outs from the fluid, metal corrosion, scaling, use of impure water for fills, etc. Happens not uncommonly, and I have probably lost more radiators on modern vehicles due to blockage than due to leaking.

Again, also remember the water flow is also regulated to the core, not just the air flow passing over/through it. Typical HVAC designs, and even the oldest C bodies had mechanical regulation valves in the heater lines.

Bangerla

They all should have the after run pump. This pump also runs when the car is running. This is the same as on the D2. The pump often dies, and then the coolant flow through the cores is reduced. The pump is quite restrictive, so when the motor fails it greatly decreases the coolant flow. On my D2, I opened up the pump and removed the impeller, which restored the flow.

MP4.2+6.0

Where is the after run pump found on a D3 4.2 I have never been sure on my W12, but I get the sense it is accessed underneath for that W12 motor.

If it has the after run pump in a 4.2 (there was none in my 2000 C5 A6 4.2), then it would probably have the check valves that tie to not pumping water backwards through the heater core in after run mode. Those could obviously get blocked or screwed up too...or lines or valves reversed if work was done in the relevant area and water lines disconnected at some point. That also reminds me for OP that on the heater valve block, IIRC reversing the lines there is a no no too. Could be the check valves are placed there. Thus another question of whether any work was ever done in that area where lines could get removed and crossed.

Bangerla

The coolant pump is integrated into the heater valve block.

MP4.2+6.0

I think from reading Bentley that is an auxiliary pump (V50) built into the valve block unit that works during engine operation to boost flow, probably at low water pump speeds. Per Bentley, the after run pump (V51) indeed is down under the motor (on a W12) basically, jammed in near the subframe. I looked for it recently when I was under there doing a tranny fluid change and thinking about my next coolant change at the same time, and could just barely see what I can now pretty much confirm is what Bentley seems to be talking about--it's described in the engine mechanical section, while the valve block and integrated pump is described in the HVAC section. I didn't see a reference to this other after run pump for 4.2's in a quick skim of the engine mechanical areas. You have to pull a lot of stuff out and support the motor to get at it on a W12.

Net, from glancing through Bentley again, I think this after run pump is a W12 thing (and then so would be the related check valves). The W12 SSP also discusses it, but only with block functional diagrams and not an actual component layout. But meanwhile the valve block that regulates hot coolant flow and has an auxiliary pumping function remains relevant to OP's issue with poor heat from both our perspectives--thus, need to check its function, and for any reversed wiring or hoses if any work has been done in that area. Bentley is specific that if things get reversed, that auxiliary pump function ends up fighting the normal water pump related circulation system.