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Old 08-04-2013, 12:46 PM
gary@germanautosolutions's Avatar
gary@germanautosolutions gary@germanautosolutions is offline
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Location: Grand Rapids, Michigan
Join Date: Jul 2010
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Mein Auto: E39 528i, E60 M5
I can't 100% certain, but from my analysis of the system I've learned the following:
  1. The cap has a blow off valve (OEM calibration of 2.0 bar, GAS calibration of 1.2 bar) that vents excess pressure through passages in the perimeter of the cap.
  2. The cap has a 1-way check valve that allows air to pass in the opposite direction if the expansion tank pressure drops below atmospheric pressure during cool down.
  3. The little hose connecting the top of the radiator to the top of the expansion tank helps to bleed off trapped air in the top of the radiator. I also believe, based on the flow path of that hose through the pressure relief section of the coolant cap, that it also makes sure that an overfilled system will vent excess coolant, and after successive heat cycles, return the coolant levels to normal. In a normal system there is coolant flow through that hose and if the system exceeds the blow off pressure of the cap, the cap will vent both trapped air from the top of the expansion tank and coolant flowing through the little hose. That is why you see coolant residue on the top of the expansion tank if the cap vents. If the cap only vented trapped air from the top of an overfilled expansion tank, the system would over pressurize every time the car was run and the system would remain overfilled.

"I'm a little rascal on my Little Rascal"
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