What's inside that header tank?
 

Before I tossed my old header tank, I decided to saw it open to see what sort of structure was on the inside. It's a bit more complicated than I expected. Photos below. The right side of the tank is a honeycomb of plastic pockets. Water entry is limited by a small hole in the bottom of each cell, and a small hole in the top which prevents the cell from ever being completely full. This traps a bit of air, which is what allows fluid expansion without fluid loss. You'll notice that the air pockets are filled with black goop, which seems to be a combination of oil and algae. As you can see in the last photo, these cells never really fill. The new tanks have something rattling around inside. I expect its a silicate SCA (supplemental coolant additive), which gradually mixes with the coolant. A very interesting design.

Nice one - no wonder they are so bloody difficult to clean! I've been toying with the idea of water and baking soda to try and get mine a bit more respectable.

Stretch, let us know if you figure out the magic solvent.

My concern is how that solvent will change / damage the silica pack. Later cars with the silica pack are supposed to get a new tank / new pack after a citric acid flush, unless the tank is temporarily replaced for the flushing process.

This structure is greatly beneficial to keep the tank structure intact, otherwise it can bloom.

Regarding the silica pack, if you use a tank with a silica pack, please do not use any generic coolant, you need the lightly silicated G-05 only, if you add a life long type coolant, you will be quite cross to see your coolant start to get milky and thick.

I think the ribs are for strengthening the tank against implosion when under vacuum. What did you replace yours with? If I were to replace mine, I'd get one (from an SDL?) with a level float switch and wire up an audible alarm.

I don't think vacuum will be too much of a problem - the cooling system gets pressurised to a greater extent than any cooling forms a vacuum doesn't it?

You can easily make a plastic tank that will withstand pressure, the trick is trapping the air cushion to absorb expansion. The baffles are there to create an air pocket, period. If they were there for structural reasons, the holes could be much larger to facilitate filling. Because the holes are so small, it's important to slowly add coolant until it comes up to the filler neck and stays there, otherwise you'll end up low when the coolant finally settles out. You hear all the time about people "burping" their system after refilling. Who would have guessed that the burp lived in the header tank?

As it happens, I'm currently running G05. And yes, if there's actually an SCA packet in there, then the coolant chemistry has to match.

The new tank came from a 500E, which happens to have a tank that's dimensionally identical to a 190D turbo (but not to an NA 190D). I don't know what model you drive, but I believe all 190's have level float switches, which operate a dash light.

Why do you need to trap the air cushion to absorb expansion and why do you need the baffles to create an air pocket? There is an air pocket with or without the baffles as long as you don't fill it to the brim.

I still think those structural cells is for strength against vacuum, which is present when the system cools off. VW expansion tanks are even stronger, it's a round ball with internal supports and can withstand 29" hg of vacuum. The quickest way to fill the VW cooling system is to draw a vacuum on the system (all the hoses go flat), dip the suction hose into the fresh coolant, open the valve and the system is filled within seconds with no need to burp air afterwards.