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Old 03-25-2013, 05:26 PM
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bluebee bluebee is offline
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Location: San Jose, California
Join Date: Mar 2008
Posts: 25,236
Mein Auto: 02 BMW 525i M54 auto 130K
One question arose in a different thread today about the REVERSE of the venting:
Originally Posted by rdl View Post
Radiator caps usually have a one way check valve to allow air into the system if pressure falls below atmospheric. I assume the BMW cap does too.

Keep in mind that there is always air plus water & EG vapor in the top of the expansion tank. When the system cools overnight from the temperature during the bleeding process the vapour contracts which reduces pressure, perhaps below atmospheric at which point the cap's check valve opens to allow air to enter and equalize pressure. If the check valve doesn't operate properly, the rad hose collapses. The effect would be magnified if the bleed isn't complete and there is another air pocket in the head or cabin heater circuit.

I think this effect is the reason that many people report having to add a bit of coolant the next morning after bleeding the system. Once that 2nd small fill is done the system is stabilized to ambient temperature and no further addition is needed (at least we sincerely hope
So, I ask: Does the GAS 1.2 bar cap (or the original 2-bar cap) have such a check valve?
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