[fpvsportsguy]

So for the S63 engine (and maybe even the n63), the overflow tank return hose seem to be a common failure point at one of the unprotected junctions. It's at the driver's side right underneath the side cowls. One end goes to the tank, one end goes to the top of radiator. Mine had a crack and I was losing a fair bit of coolant. First I thought it was the turbo coolant pipes underneath the engine cover, which is also a pretty common failure point due to heat cycling (mine was leaking a little bit, but not catastrophically), but after fixing that I still had a leak, which led me to do a thorough search and found the leak on the tank return hose. It's a pretty simple replacement, part number is 17127599571. Just thought I'd share.