Hello,
noticed a weird problem. I replaced the thermostat and coolant a couple days ago. Was out driving (highway, ambient temperature 0C), temperatur gauge stable at 85-90C. Suddenly it started rising. At 115C I pulled over and popped the hood to find the upper and lower radiator hoses totally collapsed like there was a vacuum inside the cooling system.

Any ideas? The small line from the top of the radiator to the reservoir warms up like the radiator and there seems to be a decent flow.

Reservoir cap not working correctly, letting air out, but not back in?

EDIT: The '94 G350TD is the car in question. The other one never breaks down.

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'95 E36T AMG (124.092)
'98 E200T (210.235)
'00 E430 4Matic (210.283)

When the thermostat opens, the water pump begins to draw from the lower radiator hose, which also means that it's drawing through the radiator and the upper radiator hose. If you have a constriction when the engine is running, the usual cause is that a lower radiator hose is collapsing under suction. This is usually apparent at higher engine speeds. In fact, it's usually a puzzle, because the engine overheats at speed, but the hose returns to normal and temps go down when you pull over. In your case, it sounds like the upper radiator hose has given it up. I would change both hoses. I'd also pull the thermostat to check, since it's right there. May as well replace that, too.
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