I've been told once or twice its nothing to worry about, but there is a thin hose that goes from just behind the coolant reservoir cap and lays down on top of the wheel well. In my car the end of this hose is frayed and open, and I cant locate anything else with a frayed/open end nearby. I've read this is an 'overflow hose' but I'd think it would dump coolant on a good hill and allow water to become steam and escape over time. I know on some later mercedes models there is an 'overflow' tank connected to the reservoir, but I dont think my 300d has one.

Does this part of the coolant system have to hold a lot of pressure, as much as the rest?

Then again I opened the reservoir cap earlier, it hissed for a second, so its holding some amount of pressure.

it's nothing to worry about, the hose does not connect to anything. it is indeed an overflow hose only. I hook mine to a G05 coolant bottle, so I can monitor any fluid escaping, and draw it back in when the engine cools, but it's not needed if your cooling system is in good order.

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You are refering to this tank?

This is the Expansion tank, this is actually where you fill the Radiator. large hose out the bottom connects to the bottom of the Radiator and supplys coolant to the system. small black hose from the top of the Radiator would be an over flow hose and dumps coolant back to the Expansion tank when it get warm.

The hose from the cap just hangs down towards the ground to spill any coolant if the systyem over heats.

Keeping the coolant level and the 1/2 way mark on the tank, and you shouldn`t have any concerns to worry about. think there is a full level mark on the tank.

Charlie

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awesome, good news from both of you, i havent been ignoring something thats of consequence.

@charlie
yep, that silver nozzle on the left side of the reservoir tank iis where the hose connects that i'm trying to describe.


the design still seems strange to me, seems like ill lose a bit of coolant on hills.

No. The radiator cap normally seals below the level of the hose nipple. Coolant is only going to get to the nipple if the radiator cap relieves excess pressure.