Expansion tank cap question
Slater - 17/12/20 at 07:42 PM
I am changing my radiator from this one that had a 15 lb radiator cap.
To this one without a cap, it just an 8mm bleed port which I will connect to top of my polo expansion tank. The expansion tank is the highest
point.
The polo expansion tank cap is below, my question is will this cap allow the system to work ok? Is this a venting type cap? Does it allow air back in
when fluid is cooling down?
I'll plumb it as per nice picture below that I got off here.
obfripper - 17/12/20 at 08:40 PM
That is a 1.5 bar rated cap, it is valved to will let air in when under negative pressure.
You may want to check everything over at the new higher pressure, as it may cause leaks in previously ok joints.
Dave
voucht - 17/12/20 at 09:41 PM
I don’t think pressure rates on radiator caps are about letting the air in. They are about keeping a certain pressure in the cooling circuit. When the
pressure inside the cooling system exceeds the rating, there is a relief valve opening in the cap that lets the excess of pressure out.
Having a bit of pressure in the cooling system is important for 3 main reasons:
At atmospheric pressure, the coolant would boil at 109°C. It is of course imperative to avoid boiling in the cooling circuit to preserve the
head gasket and other components. The higher the pressure is, the higher the boiling point is, hence pressurized cooling circuit.
The higher the pressure is, the more unlikely cavitation has chance to occur in the circuit, especially in the water pump. Less or no cavitation in
the water pump = water pump working at its maximum efficiency.
Heat exchange between the radiator and the air is proportional to the pressure of the coolant in the radiator (= in the cooling circuit). The higher
the pressure is, the more calories are being taken out from the radiator by the air, so the better the cooling is.
Performance / Racing radiator caps have higher pressure rates than road cars for these reasons. For example, while your Polo cap seems to have a
rating of 1,5bars (even though it is hard to read on your picture), the very famous “Stant” racing caps have ratings up to 2,21 bars for what I know
of.
Of course, higher pressure rates need a cooling circuit designed accordingly, as they increase the risk of leaks in the joints or weak/bad components
to burst. It is, as everything else, a question of compromise.
You say your former radiator cap has a rating of 15lb, so 1,034 bars. I wouldn't worry about using the Polo cap which seems to be 1,5 bars: just
a little more pressure which is good for a slightly better cooling, without increasing the pressure in your circuit. And if it is OK for a Polo, it
should be OK for your engine 
FYI, my Stant radiator cap has a 1,72 bars rating (Haynes Roadster, Sierra 2,0i DOHC engine, Mercedes 190E Radiator), and it is just fine.
Hope that will help 
Slater - 18/12/20 at 10:32 AM
Perfect, many thanks for the very informative and detailed replies.
I did notice in the pic of the underside of the cap the figures "15", but on closer inspection it shows 150, not sure if this means 15 psi
or 1.5 bar or something else....
Anyway, I'll plumb it up and follow Dave's advice about checking for leaks.
obfripper - 18/12/20 at 10:54 AM
150kpa, same as 1.5bar, nearly all vw caps are the same.
Dave
snapper - 21/12/20 at 05:02 AM
25lb Stant cap, a mix of ally tube without sewage ridges and silicon pipes, some older jubilee clips, some new, some Mikalor clamps never had an issue
with the joints as I have a good pipe tube overlap