Air pressure outside the engine can be considered to be constant for this illustration. It does not"become" greater, which implies change.

Rather, as the liquid inside the cooling system cools, it becomes denser than it was when it was hot. As it becomes denser, it occupies less volume. Atmospheric pressure outside the system is around a constant 15 PSI at sea level.

This constant outside pressure, in tandem with the liquid becoming more dense, allows the coolant in the bottle to be drawn back into the system. Because there is now space for it.

A liquid wants to seek its own level.

 

^^^^^^Bingo, couldn't have said it better.....nothing to do vacuum inside the motor and/or the jiggle valve in the stat "breaking" the vacuum. The only thing I would change slightly is that if/when the pressure inside the cooling system becomes less than atmospheric pressure, atmospheric pressure pushes coolant from the bottle into the radiator via one of the two valves in the rad cap.

Our XJ is the 1st and only vehicle we've owned in which the factory stat had a jiggle valve. Since we no longer have a factory 195 stat installed, the new stat (Stant SuperStat 180.....cringe) does not have a jiggle valve and all is still good.

Again, the rad cap on the radiator is critical to the proper function of a "open" type coolant recovery system.....not a jiggle valve in a t-stat.

 

Can we agree upon this......as long as the vehicle is on level ground or on a incline, however slight with the front up, there's no need jack the front of the vehicle up or remove the temp sending unit from the back of the head. Also, can we agree that the radiator should be filled very slowly, stopping occasionally and briefly, then continuing to fill 'til full. The next step is to start the motor and let it idle with the rad cap off keeping the rad full to the top while the motor continues to idle. Last step is after the motor has reached operating temp and fresh coolant has "burped" out the neck of the rad, install a new rad cap.

For the next several days, coolant level in the rad should be checked/monitored motor COLD. Coolant level in the rad (motor COLD) should always be at the very top of the neck of the rad. Coolant level in the rad, among other things, should be checked as I have suggested in my signature.

No need to turn on the heater on '97 up XJs or any older XJs in which the HCV has been eliminated.

 

Stant does not have the jiggle valve but it has a v notch so it does not close off tight like the OEM.
The purpose of the jiggle valve (or the v-notch) is to bleed air AFTER engine has been re-filled with new coolant.
If it is used for another reason please explain.
You are correct on density and atmospheric pressure but that is the result of temperature change like I was making a point of. That open loop your referring to is the engine block/head only. The radiator is blocked from open loop when thermostat is closed. It (radiator) has to be part of the loop in order to purge air out of top radiator hose. The bottom radiator hose is still open to the block but you cannot purge air through a column of water with just atmospheric pressure.
No one here has stated that the jiggle valve has anything to do with the recovery tank.

 

Again, if the cooling system is properly filled, the tiny amount of air inside the motor is a non-issue and will be relieved/burped within a few heat/cool cycles. This applies to both open and closed cooling systems. As long as the cooling system is properly sealed/maintained, no air will re-enter the cooling system. However, how many times have we seen a thread saying "my XJ had a very small coolant leak when I bought it a couple of years ago but I just keep adding coolant when it gets low and it never overheats but last Saturday while off-roading bam, it overheated". There's a situation where the cooling system constantly has air in it and the damage is being done slowly and surely.

I thought I had made reference to the "V" notch vs the jiggle valve in this thread but apparently it was in another cooling thread.

Once again, no t-stat will seal water tight or air tight even when fully closed.....they all have some leakage/bleeding even when fully closed.

The only time the jiggle valve is open is when the motor is off so if there should be any air up high at the front of the head, air could pass thru the hole but only when the motor is off. A jiggle valve stat may possibly allow for an ever so slightly quicker engine warm-up, due to it having the least amount of leakage/bleeding when the motor is started and coolant temp is below the stat rating.

The "V" notch stat however, will bleed whether the motor is running or not. And once again, drilling holes in the flange of the stat serves no purpose what so ever other than to greatly slow down motor warm-up, especially in cold weather.

A closed cooling system will purge air to the bottle whether the stat is open or not. Both open/closed systems will purge air because stat do not seal air or water tight.

I believe most folks understand the coolant bypass loop and the radiator loop in both open and closed system. I can explain in another thread if some wants to know.

Why would there be air at the bottom rad hose?.....air would rise to the top, no?

I thought I said the jiggle valve had nothing to do with "breaking" a vacuum, did not say anything about the jiggle valve having anything to do and the bottle(s).

I gotta go get a beer.