Hi im been having this problem for the last 3 months now. my will slowly run low with no trace of leaks. happened about twice now. any ideas where i should look at? and yeah no white smoke or sweet coolant smell coming out of the exhaust. plugs are fine. i highly doubt its head gasket. could be water pump? watcha all think?

 

might have a leaking coolant temperature sensor

http://www.audiworld.com/tech/eng47.shtml

recently found this to be the case with my 2.8 which, compared to this DIY on what i'm guessing is a 1.8T, is a lot harder to reach

the O-ring might be shot, check under your car near the rear of the engine for drips after a good drive

even though you said no trace of leaks, either with the 1.8 or 2.8 and a flashlight you could see visible coolant crust or not and just rule it out

 

What's the oil look like? Could be leaking into the crank case somehow, maybe head gasket, oil cooler, something along those lines. If you're looking for puddles, you probably aren't going to find any. Most of the time coolant leaks will appear as a pinkish-white crust at the site of the leak instead of actually dripping anywhere.

 

I'm betting on the sensor housing at the back of the engine. Mine had a hairline crack and took me forever to figure out where the coolant was going. had it not been for some great threads on here, I may have never found it

 

found some crusting on the firewall behind the engine. about 3 inches down from the top of the firewall.. however its not pinkish at all. it is somewhat really black or burnt crusting. could this be it? also i'll check the coolant lines and other areas that was mentioned when i get a chance on my next day off. thanks guys.

 

Thats it!

http://www.audiworld.com/tech/eng54.shtml

hope you have the V6 if not then that wont help

 

http://www.audiworld.com/tech/eng76.shtml

thats too

 

You might also want to consider replacing the coolant expansion tank and cap. I was getting the same symptoms and replacing those solved the problem for me.

 

1.8 has a different approach? I have a 2001 1.8 , Thanks for all the help i really appreaciate it. I'll go order the part now.