Repair-It

I was within range of my neighborhood when I saw a low coolant warning pop up in the speedo. I didn't think much of it because I had been driving a little aggressively and just took a corner pretty quickly so I figured maybe it just triggered the sensor. Kept driving a couple miles to my driveway. As I turned into my driveway I looked down and saw that the temp gauge was pegged over 120C. While in my driveway, the A/C compressor shut off. I was able to park the car. I never saw a sudden loss of coolant, a plume of steam, nothing. Fortunately I also never saw a more angry warning commanding me to shut down the engine immediately. I assume the car has some 2nd stage warning for that. So it probably didn't get up to a damage threshold, which is good.

In fact, I parked it, went inside, changed, and came out to drive it on ramps so I could get under it. I now saw coolant everywhere. It started and ran 100% normally and the gauge was no longer pegged.
 
Needs an upper hose it turns out. It popped off.

Try A2215013582

https://www.mercedesbenzpartsshop.com/o ... 2215013582

The spring retainer rubbed against the metal hose sleeve enough for it to rust. The rust jacked the spring clip enough for the hose to pop out under pressure. Neat.

Shot from the bottom Looks like a 1 minute job from the top.

Is this the spring retaining clip?
A1409950405

No it is not.

I think it is this: A0009931202
https://www.mercedesbenzpartsshop.com/o ... ng-9931202

Fixed! Old hose was actually rusted where the retainer clip is supposed to grab.

A little WD-40 on the o-rings and it popped right in. Vacuum fill and it was done. Did a quick road test, no issues.

Little more detail on what happened here. The groove that the retaining clip holds rusted away to nothing. This is the upper end of the same hose for reference.