I learned the hard way and installed the new water distribution tube without replacing that $2.50 o-ring at the joint you have circled. Man I was bumbed!
If you need to replace that o-ring, it's a tough job because there are several fasteners that have to be removed in order to pull the pipe away from the distribution tube enough to replace the o-ring. A few fasteners......sure not bad but they were not designed for access after the engine is installed in the chassis. You have to do it, so to get started do some planning. Drive up on four ramps, develop your approach and what new tools you might need.
My water distribution tube barbed popped out just before lock down last year. The total job replacing the distribution tube and then circling back to get the o-ring was over 15 hours.

 

Yikes - 15 hours!

A couple questions for you... any chance you happen to have the part number for that little o-ring? I saw in the schematic that it looks like the AS Coolant Pipe is fastened in a couple places... what all did you have to remove to get the necessary access to the pipe and o-ring when you circled back?

 

15 hours for water distro tube and o-ring, Probably 3-4 hours on oring. You will have to remove Inner fender liner, may have to remove plastic panel under front bumper (can't remember name), cat heat shields and passenger airbox. You may be able to pull the coolant pipe without removing the fasteners up front but you will be springing the tube and I wouldn't suggest going that route. O-ring part number 999-701-825-40 Engine Coolant Pipe O-Ring, L x W x H: 0.1 x 1 x 1, I also installed an aftermarket o-ring kit for vacuum pump b/c it was loosing vacuum over night, Removing this allows good access from top for final torque. Ebay search for RKX 3.6L 4.8L Vacuum Pump Reseal Rebuild kit for Porsche Cayenne Panamera 970. It's a special square-ring, may be able to get away with a o-ring but the time wasted specing out a o-ring will be worth just getting the kit. If you are dripping coolant in the winter time from this area, this is your fix.

 

Did you drain the coolant completely when you went in the second time to replace the o-ring?

 

Oh yeah good question! By the time I had popped the barb on the water distribution tube, replaced the tube and all the coolant everywhere I had become numb to coolant being everywhere. Since I had removed all of the fasteners I could easily pull out the pipe from the disto tube, plug the hole with correct size vacuum cap to reduce spillage. Replacing the o-ring isn't bad at all......it's getting to that point that is a little time consuming. I also wanted to mention that unless yours is a pool on the floor each night kinda leak, maybe consider a few drops in the coldest days of winter and the coolant smell that goes along to be tolerable.

 

I had the same thought about trying to plug the hole and the pipe with caps and then swap the o-ring quickly... just not sure if that's being over-optimistic with how quickly that can be done in a confined space!

The leak is minor at this point but it came out of no where (albeit with the arrival of the colder winter temps), so I'm thinking I might take a shot at replacing the o-ring before it gets worse. I swung by the dealership today to pick up the o-ring so I'll see when I can take a crack at it.

 

The pipe on my 2008 S is leaking on both ends. Happened while driving and had a massive coolant loss. Indy shop here in Atlanta has quoted right at $7000 to repair. Yikes! Now have to decide if I want to tortore myself this much and DIY.