Where does cooling system vent overpressurization?

[paramax55]

I'm in the process of testing the car to gain confidence in the head gasket after a water pump belt broke. But the upper radiator hose exploded long before the temp gauge moved at all. That got me wondering (besides "is the cap good") where does the pressure dump? This seems to be a completely closed system - as in it can't relieve pressure. I mean, there is no overflow port or tube. So where does the overflow go?

[Effduration]

Expansion tank cap...It is rated at 2 bar which is about 29 psi.

Telltale signs of exhaust getting into coolant:

- test for exhaust gas in coolant turns green
- coolant hoses stay pressurized for hours after shutdown.
- coolant in cylinders in the morning after running it to temp the day before.

If I were you, I would do a exhaust gas in coolant test...free or mostly free at autozone.

[paramax55]

I've already ordered a test kit. But the excess coolant can't go into the cap. It has no volume. It has to go somewhere.

[80stech]

[Effduration]

Originally Posted by paramax55

I've already ordered a test kit. But the excess coolant can't go into the cap. It has no volume. It has to go somewhere.

I believe you are suggesting you have excess pressure not excess coolant.

Assuming the cap's pressure valve is working properly, excess coolant pressure, with some coolant, will escape through the expansion tank cap and onto the ground, or splashed around the engine bay.

Here is an AI-generated description:
" An expansion tank cap releases pressure when coolant gets too hot and expands, exceeding its rated PSI, preventing system damage like ruptured hoses or blown head gaskets by venting excess pressure through a valve to the overflow tank or ground....."

[paramax55]

Every system will generate pressure when it heats up. All cars historically had a non-pressurized expansion tank and the excess went through a drain hose to the bottom of the engine bay to drain to the ground. Does BMW really have it just leak onto the top of the tank and splash around in an uncontrolled fashion? I also do not see any holes in the cap that would let out the excess pressure.

[Clavurion]

Originally Posted by paramax55

Every system will generate pressure when it heats up. All cars historically had a non-pressurized expansion tank and the excess went through a drain hose to the bottom of the engine bay to drain to the ground. Does BMW really have it just leak onto the top of the tank and splash around in an uncontrolled fashion? I also do not see any holes in the cap that would let out the excess pressure.

If the cooling system is working correctly and coolant amount is correct the system pressure should come near the cap pressure value, but should not go over it to bleed out. Some models have a small nipple and hose on the expansion tank but still they are just guided to ground. On E53 the possible over pressure will flow on the surface of the tank.

If you check the underside of the cap you can press the pressure valve in the middle and test blow where the air is coming out. On petrol engines the cap is 2,0 bar and on diesels 1,4 bar because of different designed running temp.

[andrewwynn]

Mine had a channel off the back side of the expansion tank but no hose —!would dribble onto the wheel well.

I thought the cap had failed but it was the tank itself.


–awr–

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[andrewwynn]

Originally Posted by paramax55

I've already ordered a test kit. But the excess coolant can't go into the cap. It has no volume. It has to go somewhere.

Unless you over fill there's never excess coolant. Unlike typical American cars with an "overflow" bottle the coolant reservoir on e53 is included in the pressurized coolant loop. It's about half full to take on the normal expansion due to temp.

Pressure is also related to temp so pressure builds when hot but will not get near 2 bar unless something fails.

You can put some uv dye into the coolant to detect if the cap is m relieving pressure but the suggestion of test for pressure on the upper hose after shutdown.

You can use blue shop rags as a telltale also: they turn dark when wet.

The test kit measures the gas coming out of the open coolant fill cap. It reacts with exhaust gas and will change color if there is gas leak.


–awr–

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[oldskewel]

Originally Posted by paramax55

Every system will generate pressure when it heats up. All cars historically had a non-pressurized expansion tank and the excess went through a drain hose to the bottom of the engine bay to drain to the ground. Does BMW really have it just leak onto the top of the tank and splash around in an uncontrolled fashion? I also do not see any holes in the cap that would let out the excess pressure.

There are two fundamentally different approaches to capture fluid expansion. This video explains it pretty directly.

https://www.youtube.com/watch?v=jkhYt-OaipE&t=1s

Cars I've had with the expansion tank (degas bottle) approach:
1991 Lexus LS400, 2001 X5, 2004 Porsche Cayenne, 2012 Toyota Prius
So it is not a rare or brand new invention.

I think the Lexus and Porsche (maybe not a coincidence ) have hoses attached to the expansion tank cap that redirect any overflow to the ground, vs. the BMW approach which is to spray coolant everywhere to be sure things start smoking under the hood and the driver is alerted.

The expansion tank is also called a "degas" bottle or tank. One purpose of it is to capture air and water vapor in the system. So it is at the high point of the engine + cooling system. Any bubbles that pass through the tank should get captured there while bubble-free coolant continues through the system.

As others have said, unless there is a failure of some sort, the expansion tank release valve should never open. This is unlike an overflow tank system, where the radiator cap release valve pressure setpoint sets the pressure of the cooling system in normal operation (expansion that would increase pressure above that releases a controlled amount to the overflow tank, keeping pressure regulated at that setpoint). If normal operation in the BMW give a pressure of about 1 bar, whether you have a 1.2 bar, 2.0 bar, 25 bar, or infinity bar cap, the operating pressure and temperature will not be affected UNLESS there is a cooling system failure. At that point, the design is intended to vent coolant while saving some more serious explosion due to pressure buildup.

You can get more info if you google for
overflow expansion tank automotive degas cooling system

M54 cooling system: