Can someone explain the flow through the expansion tank so-called "overfow" hose? - Bimmerfest - BMW Forums

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E39 (1997 - 2003)
The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

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Old 08-04-2013, 03:52 PM
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bluebee bluebee is offline
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Can someone explain the flow through the expansion tank so-called "overfow" hose?

I open this thread simply to discuss the flow of air & fluid in the so-called "overflow" or "bypass" hose connecting the BMW E39 expansion tank and radiator.

Deep into the GAS 1.2-bar cap discussion, in post #84, Gary explains how the expansion tank pressure cap works in the three situations of proper filling, overfilling, and underpressure.
> E39 (1997 - 2003) > Got the 1.2 bar expansion tank cap from German Auto Solutions

But, I don't wish to clutter up his pressure-cap thread with overflow-hose questions, so, I bring the question out to this thread.

Q: What is the purpose and intended flow through the hose connecting the expansion tank & radiator (under normal and over/under pressure situations)?
Here's Gary's explanation, for starters (but the hose flow still confuses me).
Originally Posted by gary@germanautosolutions View Post
I can't be 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.
Since the hose is attached the highest point in the system, and has a bleed screw port, it's the last and best place to bleed any remaining trapped air in the system.

The passage that the hose connects to enters a separate circuit between the two o-rings on the coolant cap. This is the return path of that hose flow into the expansion tank. Since this hose is passing coolant during normal operation, and that coolant passes through the cap just below the pressure relief valve, if the system pressure exceeds the cap rating, when the cap vents that pressure, it will vent both air and coolant.

This system helps to vent any steam first if the system has actually overheated, because the stream will enter the system at the top of the radiator. But also helps to rid the system of excess coolant if the excess pressure is caused by overfilling the expansion tank.

Please read the suggested threads and add value, either by pictures or by descriptions, so the next person stands on your shoulders.
See also: E39 Bestlinks & How to easily find what you need

Last edited by bluebee; 08-04-2013 at 07:16 PM.
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