Bart247

There is a tube with a cap on it near the oil filter on my 2007 S8. It comes up from behind the engine and loosely sits between the oil filter and EVAP valve. The tube has a black cap that is easily pulled off like a plug. There was no fluid at the bottom of the cap or on the inside of the tube, it's dry and looks empty.

Does anyone know what the purpose of this tube is?

mishar

As far as I remember all S8's have that tube, but I am not sure that anybody knows its function. Could be just Art of Engineering.

Mister Bally

Any chance that it is connected to the exhaust manifold? My older '87 and below Audi's and VW's had something like this to measure CO ahead of the catalytic convertors. The tube was stainless and the cap was Teflon though as the tube got warm and the gas was hotter than hell.

Bart247

I went through my repair manual and the best match I could find is the Transmission ventilation hose.

The cap at the end of this hose popped off very easily. The problem is I dropped it and it fell behind the engine. I took off the plastic belly pan under the car hoping it made it all the way down and landed there, but no luck. I tried my endoscope but it's hard to get far angles with such a tightly packed engine.

For the brief moment I held the cap it looked like the bottom had some vents and the fact that it wasn't on so tight leads me to believe that it may vent some gas.

What do you suggest I do? seal it closed temporarily until I get a replacement cap or leave it open?

Bart247

I found the cap

It's called a Automatic Transmission Dust Cap.

VW Skoda Aodiming Rui Hao Sagitar Maitengpasate Automatic Transmission Vent Hose Dust Caps-in Other Transmission System Accessories from Car / Products / Accessories / Modifications on ChinaOntrade.com

MP4.2+6.0

Back on really old ZF trannies I removed from Audis, there would be an air vent at the top of the transmission case. It would have a small metal cover crimped on to it that you could move to the touch. Of course, this was also in the days of dipstick based check and fill systems and three speeds.

This sounds like the same thing, other than they have put an extension hose onto it for some reason. Maybe it is either to get it a bit further away from the heat that would be back there w/ the exhaust just above, and/or to get the air outlet effectively higher up if the car were ever even momentarily submerged up to the level of the transmission tail. The cover is presumably to minimize dust ingress, or water or other stuff getting sprayed int it somehow from above.

Bart247

Should I leave it open until I get a replacement cap, or cover it?

MP4.2+6.0

I would not plug it up, but if you want to you could do something like simply folding over a piece of duct tape loosely over the end but where it is still not fully sealed up.

Bart247

I'll do the duct tape.

Thanks.