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Guys, if the upper hose had sprung back to its full round shape,
Back up ... who said the upper hose sprung back into a pleasingly round shape when the sensor is removed?

Is this something you/anyone observed?

I agree that it's the ambient air pressure that is keeping the coolant in place (same thing with a drinking straw holding water), but air does not necessarily need to displace lost coolant (assuming the system is truly sealed) instead the air pressure in the upper hose/expansion tank could just be reduced if coolant escapes from the sensor hole.
 

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Back up ... who said the upper hose sprung back into a pleasingly round shape when the sensor is removed?

Is this something you/anyone observed?
normally...remove and replace, I don't mind losing a spoonful of coolant. I use a rubber (or cork) stopper so as I can take my time. sure way, no air can come in.

I think the squeezing trick is to negate the tendency of flowing out, not to create a tremendous vacuum to suck everything in the vicinity

I haven't done this trick myself, just curious, and might try, just to see if/or how fast the upper hose would return to its glorious state.
 

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Open up the radiator (expansion tank) cap half way. Squeeze the upper hose with all you've got.
Back up ... who said the upper hose sprung back into a pleasingly round shape when the sensor is removed?
After MrM squeezed the hose with all his weight on it, then what kept the hose from jumping back when there is a nice opening hole on the lower hose? Even if the upper is not returning to a perfect round, it must had sucked in some thing or it is a dead duck.
 

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Discussion Starter · #45 ·
there would still be coolant immediately occupying the area preventing air to come in, unless ambient air pressure is enough to overcome.
To be honest I don't think I was going to loose anything even if I didn't squeeze the hose,i could be wrong but not even a
Wait a micro-second, there is no coolant floating in the air for it to suck in through the sensor hole. You're driving me crazy here. Where was the coolant that being sucked in via the sensor hole?
Correct nothing got sucked In. It was a 10 sevond job, what is the major thing with this lol. It worked it saved me money and I'm happy. :)
 

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After MrM squeezed the hose with all his weight on it, then what kept the hose from jumping back when there is a nice opening hole on the lower hose? Even if the upper is not returning to a perfect round, it must had sucked in some thing or it is a dead duck.
Try it and you'll see ... it's like water being held in a drinking straw with a finger covering the top opening.
 

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I understand, but the drinking straw doesn’t have a squeezed rubber bulb on its end.
An eye dropper does, same principle.
 

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An eye dropper does, same principle.
Try it and you'll see ... it's like water being held in a drinking straw with a finger covering the top opening.
I'm not debating about if water being held or dripping out, but I'm saying air being sucked into the system
Do this: fill the eye dropper with water (and with some air in the bulb if you like to emulate the air in the tank), then squeeze the bulb flat as MrM did, then seal up the tip with finger and release the flatten bulb and it should be held flat by vacuum just same as the upper hose. Now point the dropper tip down and release the finger on the tip (same as removing the sensor). What happens? The bulb sucked in air and returned to its round shape.
 

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I'm not debating about if water being held or dripping out,
not debating, not sure if u mean agreeing being held. if being held, then coolant is occupying the area leaving no space for air to squeeze in.
Do this: fill the eye dropper with water (and with some air in the bulb if you like to emulate the air in the tank), then squeeze the bulb flat as MrM did, then seal up the tip with finger and release the flatten bulb and it should be held flat by vacuum just same as the upper hose. Now point the dropper tip down and release the finger on the tip (same as removing the sensor). What happens? The bulb sucked in air and returned to its round shape.
not representative of the cooling system. hole of the eye dropper is at the tip..end. hole of the sensor is not. there are coolant upstream and downstream of cooling system both sides of the hole where coolant can come from, ready to occupy the vacant space your air is trying hard to get into.
u need to cover the tip of the dropper, and make a hole midway to come into a more representative observation.
 

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if being held, then coolant is occupying the area leaving no space for air to squeeze in.
This is not true. Even with coolant occupying the area -- like water in tip of the eye dropper -- the vacuum inside still can pull air bubbles in through the coolant at the sensor hole (or the water in the dropper tip).

there are coolant upstream and downstream of cooling system both sides of the hole
Imagine the eye dropper has a bent tip like the umbrella handle, this makes no difference on how air being sucked in when the squeezed bulb is released. I think the bent tip is equivalent with the sensor hole?
 

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Let's assume we have a sealed cooling system here. Let's also assume the upper radiator hose is slightly collapsed indicated a slight vacuum in the system. When you remove the temperature sensor in the lower radiator hose, effectively we have 14.7 psi of ambient air pressure on that small opening. In a physics text book, this would say the air pressure would push the coolant at the small opening back into the cooling system as the system has a slight vacuum. But, don't forget the coolant in the cooling system wants to flow out due to gravity as the opening is practically at the lowest point. Also keep in mind that the opening is small, the volume of coolant is large and there are mechanical blockage of the backwards coolant flow at the thermostat. In the few seconds that it would take to actually replace the sensor, the amount of air that would be introduced into the system is practically zero.

As a physics homework, anyone is welcome to calculate the static equilibrium here with coolant wanting to flow out the opening due to gravity, the slight vacuum in the cooling system, the ambient air pressure pushing the coolant back into the system, the mechanical blockage of the thermostat, and all the other minute leakages in the entire cooling system that have not been talked about.

In re-reading this entire thread, at no time did the OP describe the upper hose being slightly collapsed and then relaxed later. This conversation went down the path of theoretical scenario, which does not help the OP. If it makes anyone feel any better, you are right that if the upper hose was slightly collapse and then relaxed later, something had to have occupied the "space". It would most likely be additional air that was introduced into the system. But in this thread, that did not happen to the OP.
 

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In re-reading this entire thread, at no time did the OP describe the upper hose being slightly collapsed and then relaxed later.
Open up the radiator (expansion tank) cap half way. Squeeze the upper hose with all you've got. At the same time tighten the cap.
OP: - Great. Your a star mate. Didn't fancy draining the whole cooling system. Thanks alot really appreciate it 👍
If OP followed Mr.M suggestion then the hose was not just slightly collapsed but it was squeezed the last drop of ...out .So the vacuum is much stronger than the slightly collapse hose from the cold temp in the morning.

The air probably not traveling through the lower hose with a closed Tstat, but it moves through the radiator and up to the upper hose.
 

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The air probably not traveling through the lower hose with a closed Tstat, but it moves through the radiator and up to the upper hose.
Someone needs to calculate the fluid mechanics of coolant/air through the narrow passages of the radiator. I'm fairly certain that is like sucking water through a narrow straw. What is the negative pressure on a fully collapse upper hose? I don't know what the exact answer is.
 

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What is the negative pressure on a fully collapse upper hose?
Everything can be calculated based on the distance from sensor hole to upper hose, but we don't know the negative pressure exerted by the squeezed upper hose.
The size of the radiator channels only affect the rate of the fluid (air and coolant) motion, but it doesn't affect the pressure at the opening or or if air being sucked in or not.
 

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Everything can be calculated based on the distance from sensor hole to upper hose, but we don't know the negative pressure exerted by the squeezed upper hose.
The size of the radiator channels only affect the rate of the fluid (air and coolant) motion, but it doesn't affect the pressure at the opening or or if air being sucked in or not.
The capacity of the M54 cooling system is approximately 8.8L per Bentley manual. I don't know how much of that is actually in the radiator, but for argument sake, let's assume 1/2 of that is in the radiator. 4.4L of fluid, with the aid of gravity is trying to rush out the open lower radiator hose opening. You can do the math exercise, but we can calculate how much additional vacuum is needed to actually suck enough of that coolant back up the radiator in order to introduce air. Do you think the additional vacuum that one would introduce by squeezing the upper hose is going to overcome the head (liquid pressure) of the coolant in the radiator due to gravity? I think we know what the answer is.

In addition, you're neglecting the frictional forces of the coolant against the sides of the radiator channels if it were flowing. The more narrow the channels, the more surface area, the more friction. Let's not forget the whole idea of a radiator is to have high surface areas as it's basically a heat exchanger. This same friction causes turbulence from the flowing fluid. All this affects the flow, as you indicated, but given the premise of this thread, which is whether air would be sucked in or not in the few seconds that it would take to replace the sensor, it matters a whole lot because flow rate = volume*time. If flow rate decreases, then volume decreases with a given amount of time. Therefore, in the time that it takes to replace the sensor, if flow rate decreases, then (if) air does get it, it would decrease. This in combination with the affect of gravity on the radiator fluid lead me to believe the additional vacuum introduced by squeezing the upper hose does not increase materially the amount of air that may get introduced into the cooling system.

Food for thought. If one were to wait t=infinity, would the upper radiator hose relax to it full shape in this scenario? I would bet a cold can of beverage that it would not. There's just simply too much head from all the coolant in the radiator for fluid to travel upward into the radiator simply due to a collapsed upper radiator hose. Who's up for doing this experiment?
 

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Someone needs to calculate the fluid mechanics of coolant/air through the narrow passages of the radiator. I'm fairly certain that is like sucking water through a narrow straw. What is the negative pressure on a fully collapse upper hose? I don't know what the exact answer is.
I've calculated that I've been doing this for over 30 years (I've worked on other cars before the E46 came about) I haven't had a single issue.

That's the mechanics of a technician.
 

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If OP followed Mr.M suggestion then the hose was not just slightly collapsed but it was squeezed the last drop of ...out .So the vacuum is much stronger than the slightly collapse hose from the cold temp in the morning.

The air probably not traveling through the lower hose with a closed Tstat, but it moves through the radiator and up to the upper hose.
Could you, would you open up a radiator cap and squeeze the hose if the engine was hot enough for the thermostat to be "open"???
 
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