marlinspike

I happen to have an expansion tank laying around here, but I'm in the middle of moving. The pinhole leak is in the top of the expansion tank, so there's no coolant coming out, just air. Also, the coolant never gets over 82C, so it's not like I need the boiling protection of the higher pressure. Can it wait a few days?

HEISENBERG

I would say its OK unless youre driving to Canada.

Fabrik8

Why is this an issue? Expansion tanks aren't sealed, and don't hold any pressure. The only thing that matters is that it holds liquid and houses the rubber tube from the expansion port of the radiator. If the pinhole is at a level in the tank that liquid will never reach (like at the top) then there is no reason it ever needs to be fixed. If the hole is somewhere that the liquid will reach, just fix or replace the tank at some point. It won't matter as long as it can hold enough fluid for normal use.


Are we talking about the same thing here?

TiredToyotaDrvr

I think he's talking about the coolant reservoir?

If so, +1 what Fab said... the tank itself isnt sealed, just the tubing from the radiator to the reservoir cap, and down into the reservoir itself...

Elaborate please

marlinspike

On an MB, or at least the older ones, the expansion tank is in fact sealed and on this car the cap pressurizes the system to 1.2 bar.

Fabrik8

What is this expansion tank made of? Metal?


Actually, is this an expansion tank or a header tank?

Fabrik8

Does it look like this:




That's a header tank, and I'd assume it's probably at the highest part of the coolant system. That overflow hose probably goes to a regular overflow tank like most cars have. So I see what you're getting at now.

marlinspike

Yeah, that's the thing. Mercedes calls it an expansion tank, so I do too...dammit!

On the later cars, the overflow hose goes to an overflow tank. On my model, it goes to atmosphere, but it is positioned such that nothing can go out of it until the spring loaded thing on the cap opens up (i.e. it's above the seal).

The nice thing about this design, btw, is you can top it off hot - just open the cap to the first notch and it bleeds the pressure.

Fabrik8

Yeah, that makes sense. System pressure is system pressure, so whether the vent tube goes to a reservoir or the atmosphere only depends on the type of tank. That type of setup makes it nice and easy to bleed the system, and there usually isn't a radiator cap on the radiator itself because it's not needed (Subaru actually uses one on the radiator and one the header tank). Interestingly, it really doesn't matter if there is air in the header tank because the tank isn't in the circulation path between the radiator and the engine, therefore air from the tank doesn't enter the cooling circuit.

This kind of stuff fascinates me to think about, sorry. It's fun to figure out the logic behind the different configurations of stuff that different manufacturers use.

Well, if there is a pinhole in the tank, you're not going to build system pressure. That means you're not going to be able to cool as effectively, because the principle behind higher-than-atmosphere cooling system pressure is to raise the boiling point of the coolant. That being said, as long as the coolant doesn't reach sea level boiling temperature then you don't need to worry about the pinhole in the tank for now. Being winter, that's probably not an issue if you're not hammering the engine under load. Hard to say though, because the coolant has to remove a lot of heat right at the top of the cylinders around the combustion area.


Do you have one of the plastic tanks or one of the metal tanks? If it's one of the metal tanks, the one in the picture looks like brass and can probably be repaired by brazing or soldering. If it's plastic, it should be able to be thermally welded by someone who specializes in that sort of stuff.

I see the one in the picture has a level sensor in the side of it, and if that isn't needed you can replace yours with just about any type of catch can or header tank that can hold what you need. I don't really see any need to buy another stock one if you can't see through the damn thing anyway. Why they didn't just put a sight glass on the stock one instead of a level sensor I can't say..

Anyway, if it's easier to use an aluminum one instead of trying to repair the existing one or sourcing another stock one......

Fabrik8

That's funny, it's the exact same thing in plastic instead of brass..






Wait, wait.... That one in that picture is plastic, it's just BROWN...!