This one has me scratching my head
 

Ok, any insight or ideas here are greatly appreciated.

Friend has a 96 12v auto quattro, all maintenance up to date. The car has ~130k miles. Now that it's solidly cold here in NY (highs in the teens, lows around 0, windchills dipping below -10), he's losing the heat in his car. Gauge will climb to the normal point if the car is sitting still but drops once he starts driving. The heat works well sitting still but fades as he drives.

Slam-dunk case for the thermostat being stuck open, you'd think, except that was changed this weekend, new G12 added, and the system burped five times. The problem persists.

In the last month, just to let you know, the car has gotten a new coolant temp sensor and has had the heater core forward and reverse flushed. Ignition timing is correct.

I know it's possible that the new thermostat is bad out of the box, and although that's rare, it's not unheard of and the persisting symptoms seem to support that. I can tell tell you confidently that the labor was done correctly - the two guys who did the work are very familiar with these cars and are two of only four or five people that I'd trust implicitly to do work on a B5, no questions asked.

Any ideas to offer? Please throw 'em out there.

he's just going crazy. did you guys take the bleed hose off on the heater core and bleed it? does the blower motor lose speed?

I've always heard that it's common for thermostats to be bad out of the box.

Another vote for a bad-out-of-the-box thermostat.

thermostat is the only thing that makes since to me maby you put it in the wrong way?

Yeah like I said, t-stat being bad out of the box still seems most likely so I agree with you guys on that. Andrew, as for it going in wrong, not a chance - the two guys that did the work have a combination of 40 years of vehicle repair, maintenance, and modification. The t-stat was installed correctly (which I did ask them as a doublecheck, although I knew the answer without asking the question) - those local AFers on here who have worked with us can attest that Bill and Keith know how to do a t-stat in their sleep and that an install screwup isn't in the cards. A defective part is the only thing that makes sense -every other base has been covered and the symptoms are classic of a t-stat being stuck open. I was hoping for some quirk that someone may have come across that could also explain it but it's not looking like it. I appreciate the input and will pass along the bad news. Service position coming up...again lol.

agree.. bad t-stat. Mines been bad for a month now... it cold :(

As for the temp gauge changing from idle and high speed thats normal during cold weather, at least i thought so, considering the cold outside air is blowing past the radiator while moving vs. idling. The fact that the interior temp. changes while moving takes me back to a 97' A6 I went round and round about same thing as you, with the thermostat. What I finally came across after endless hours of pain and disapointment was that the mixture flap for recirculate/outside air was stuck in about 1/2 way. At idle air wasn't being blown past the mixture flap so the heater core would put out enough heat, then when the car was moved the cold air would blow in through the mixture flap and make the air cold. Just a suggestion, hope it helps... At least thats how i remember it...been years tho.

Sounds exactly like my car did when I first got it last year.

It was the T-stat.

This is very interesting that you say this, I know the guys at my local Audi dealership and when he grabbed me a new T-stat off the shelf he wasn't sure if it was used. I believe this can be a common issue.

I would use a OEM if that is not what you used, but I assumed you did the coolant and used a NEW gasket?

Do you have a Vagcom to look for some codes?

Another idea, maybe the radiator?