kday

Last night I got the A6 motor buttoned up and left it idling while waiting for the lifters to stop clattering. I had filled the expansion tank with the bleeder screw on the coolant pipe open until coolant started coming out. After maybe 20 minutes the tank violently boiled over. The lower radiator hose was still cold -- i.e. the thermostat wasn't seeing the hot coolant and opening.

This was resolved by 1) turning the car around, so it was pointed DOWN the ~10% grade of my driveway, rather than up and 2) revving the engine to 2500 RPM a few times.

Nothing terribly surprising here, except I wasn't expecting it

austinado16

It's the holding them at rpm that creates a nice pressure wave of water, while at the same time, bringing everything up tempurature fast. Including noisey lifters.

Otherwise, it's just a battle.

Audi Logic

live and learn huh

mr sarcastic

noted. cause im sure tools would be put through windows when this unexpectedly happens to me.

hammer 10

Back in my younger hot rod days we used to drill a 3/32" (.093) hole in the thermostats so the engines wouldn't air lock on coolant changes. On cold mornings our Audi engines would probably stay in the cold start mode a bit longer but I can't see where such a small hole would change much. Then again if it was a good idea you would think Audi would do it. Comments, pro or con are welcome. ~ John Buchtenkirch

kday

It didn't seem to be sufficient in my case.

For comparison, 5 cylinder thermostat on left, 6 cylinder on right:

<img src="http://www.boost-instruments.com/ultrameta/tstat/tstat1.jpg">

<img src="http://www.boost-instruments.com/ultrameta/tstat/tstat2.jpg">

austinado16

But like kday points out, Audi's done it for us with that fancy little bubbler valve.

hammer 10

Well, I can tell you for sure that the German thermostat I got from World-Pack didn't have that valve as well as the one I took out. Dammed fake parts got me again. I wonder why that little valve appears to be a check valve ? Now I feel bad that I didn't drill the hole when I had the chance, I got nervous about re-engineering my Audi and didn't go with my gut feelings. ~ John Buchtenkirch

kday

And for what it's worth, I was given the one on the right at the dealer parts counter when I asked for a thermostat for my 5 cylinder car, which mounts the thermostat down low.

rmccomiskie

The t-stat I removed from my 96 A6 had a check valve. Replacement had it too.