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So I was pressure testing the cooling system of my car with a kit where you fix a cap, a gauge and a hand pump to the expansion tank and then pressurize the system to 1.5 bars (mostly European cars go that high), then you look for leaks.

Once you finish there is a release valve to let the pressure out. Even if I take care and let the pressure out reasonably slowly, say back to normal in 30-60 secs. rather than just letting it out as fast as it could, 3-5 secs I would imagine if I held the release valve fully open.

Now, when I detach the pump and gauge from the adapter over the top of the expansion tank, if I went a little too quick, liquid (roughly 50:50 mix of water and coolant) will start to flow up and out the top of the adapter. This suggests that even though the pressure was showing as zero it wasn't quite zero or more likely I let all the pressure out but as soon as I released the pressure release valve pressure started building up again and then as soon as took the valve and gauge off it let that new pressure out that was too small to register on the gauge.

The initial conditions are that the total volume of the system is about 10 liters and there is an air gap in the expansion tank of about volume of 10x5x8cm where the vertical space is the 8cm, roughly.

My guess is that this is something like when divers get the bends where gas is being infused into the liquid and then coming out again once the pressure is released.

The only problem I have with this is that the only place gas is, or at least should be, is right in the expansion tank so the only gas I could see getting into the liquid is in the tank so I don't see any need for fluid to be coming up out of the tank. That is, I would have thought that the air gap at the top would have given space for just the gas to come up through the top.

I guess a case could be made that gas being infused into the liquid made the liquid volume high enough to get to the top of the tank but we're only talk about a little bit of gas being pushed into about a 10x5x8cm space to push the system up to 1.5 bars. It just doesn't seem to me like that would be enough gas to do that but then again I don't know the math so perhaps that's what's happening.

Anyway, does someone know for sure what is happening?

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  • $\begingroup$ "...roughly 50:50 mix of water and coolant)"??? $\endgroup$
    – The Tiler
    May 29, 2023 at 8:57

2 Answers 2

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Your cooling system contains flexible rubber hoses. when you pressurize the system, these hoses stretch out a bit and the volume of the system increases slightly. when you release the pressure, the rubber hoses contract and the system volume decreases and any excess liquid flows out.

Because the rubber is viscoelastic, its contraction happens slowly which makes the system dribble excess fluid for a while after you release the pressure.

It is also possible for air to get trapped in portions of the system, which would be scavenged out by the water pump while the engine is running and probably wind up in the overflow/reservoir tank. it is typical to "top up" the cooling system after replentishing the coolant by first running the engine briefly, and then popping the system open to release the air- and then adding extra fluid as needed.

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  • $\begingroup$ The problem with the first answer is as the hoses come back to their normal size there's no reason for them to contract to smaller before it was pressurized so I don't see this being a possible solution. On the second one, the testing was after the car had been run, bled, and then cooled down to room temperature so I don't see there being appreciable air in the rest of the system unless I did a poor job at bleeding the system. But, even if there was air in other parts of the system, it would compress with pressure and return to it's original size on release I would imagine, not add volume. $\endgroup$
    – Reg
    Jun 2, 2023 at 21:00
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The system is under pressure when it is sealed. The boiling temperature of water and/or glycol increases with increasing pressure. When you release the pressure, suddenly the boiling temperature of the coolant drops and may boil.

When the system is cold and begins heating up, it remains at atmospheric pressure until you hit the boiling point of the water/coolant mix. Once a local pocket of coolant boils, it increases the pressure of the system since it expands as it turns into a gas. As the pressure increases the boiling temperature also increases of the remaining coolant.

So when you open up a hot system, the pressure will try to escape and simultaneously it will cause the boiling temperature of the coolant to drop, so you may immediately boil the coolant and cause what appears to be additional expansion.

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  • $\begingroup$ That's all well known but I think you missed that that system was never heated up. I pressurized it with a gauge and hand pump at room temperature. Waited an hour or so to see if it would lose pressure. And then released the pressure all without running the engine. The temperature never changed except for whatever changes in ambient temperature there was over the hour or so. However, I can do this in the space of a few minutes and the result is the same so I don't see ambient temperature change being a factor. $\endgroup$
    – Reg
    Aug 19, 2023 at 7:29

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