When changing out a front shock, (turbo side) I noticed a rubber line (along with a random nut) running into the spring from the top. It was cut off, and visibly wet, at one end. Tracing it up, I found (to my horror) that it ran to the coolant expansion tank (the hose on the back, near the cap, not the one leading to the radiator).

I reckon that this is a bad thing.

Because my car was running hot over the summer, I've been monitoring the coolant level, and I haven't seen any marked decrease- when cold, it sits right at the Kaltwasserstand. I also don't know too much about the cooling system.

What I do know is that water (coolant) runs through the engine block and radiator, exchanging heat with the engine in the block and with the incoming air at the radiator, cooling the block. I also know it gets right hot, 82-105 degrees Celsius, which means it should be expanding and maybe even boiling at 1 atm. Since it's a closed system, it pressurizes instead, hence the expansion tank. And since my system isn't actually closed... it's bad.

To make a long story into a short question, where should this hose be going?

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1992 300D 2.5T 249k - Parked with a bad transmission
1981 300SD 142k - Daily driver