Maybe I have something like this
https://youtube.com/shorts/qSipz__ip...3HsN0EzubzVhju

But at the rate I'm loosing coolant, I feel I would still have something dripping on the floor.

I understand the initial overheat. The system had air in it, which caused it to overheat, thus building the extra pressure.

This in turn probably blew the hose from under the factory clamp on the lower radiator hose.
So basically due to building pressure, the weakest link gave up.
If the hard tubes were leaking, I would've expected them to give out first. This is quite a challenge to figure out...

Remove the lower plastic air dam. I'm surprised how much water it can hold before dripping. It's how I confirmed a coolant leak. Mine currently leaks at the transmission oil cooler. If the transmission lines move too much, the cooler teeters. When the cooler teeters off center, the lower seal comes out and leaks.

This one?

https://i.ibb.co/ZxF7Kc2/Screenshot-...838-Chrome.jpg

Removed it right away to make it easy to look for leaks but still nothing.

I need to buy a pressure testing kit or smth, just not sure which one will fit bmw expansion tank

This thread shows a couple of homebrewed solutions to pressurize the cooling system.

https://xoutpost.com/bmw-sav-forums/...ystem-e53.html

Yes, remove the "belly pan". Get this test kit from Harbor Freight when it's on sale. The yellow adapter fits the X5 tank perfectly.

Thanks skewel, I'm going to look if I have an extra cap laying around.
I can probably just use an electric tire pump to pressurize it?
Testing through the bleeder screw would be easier, but I'm afraid that the cap will depressurize the system before enough pressure build?

I guess my reservoir cap is rated at 1.4 bar, which is 20 psi, and only 15 psi is needed to pressurize the system for testing, from what I read. So I would be good to use the bleeding screw

Being in canada, the easiest way to purchase a kit would be on Amazon.ca but a lot of them have poor description, so it's hard to tell which one has a bmw adapter.

In the meantime I'll try this

https://i.ibb.co/H7XjFH4/20240406-104541.jpg
https://i.ibb.co/Kxwzx18/20240406-104527.jpg

I think I found a minor leak, which maybe looked minor, but was major when the vehicle was running lol. My own doing, where I spliced a heater into the lower radiator hose. Tightened the clamps, and fixed the leak as far as I can tell for now.

Then I pressured the system to about 20 psi.
If I push past 20 psi, I can hear the radiator cap releasing the air. So my ryobi tire inflator gauge must be pretty accurate.

While looking for the leak I found my own radiator cap which flew away lol. It was actually maybe 1-2 years old only. Interestingly, with this radiator cap, I could hear air escaping even if I only pressurized the system to 15 psi. So I think this cap is defective, maybe because the system overheated originally with this cap on it, and the overhead was at least to 140 Celsius, maybe the cap got damaged.

The other cap holds the air at 20 psi, no leaks at all.
I listened but could not hear any air escape. That's with a cold engine.
After 10 minutes, psi dropped from 20 to 15 psi.
After 20 minutes, pressure was down to 10 psi.
After 30 minutes, pressure was down to 7 psi.
I'm not sure if that's normal, but back pressure is slowly releasing back through the pump.

One constant in all of this mess is driving.
Took it for a drive, after about 15 min, temperature slowly rising. Let it climb to 110 degrees, pull over with vehicle running, stay idling, temperature dropped to 93C slowly. I raised rpm to 3000 and waited a few minutes, temperature is climbing past 100 again.
Let idle for another 15 minutes, temperature down to 95C.

I check the aux fan, it's not running. So I know it's not the aux fan cooling things down.
Temperature outside +4C.

This is weird. I have a situation where I'm overheating when driving, but normal when idling.
I found a ton of topics "overheating when idling" but not overheating when driving, but not overheating when idling", like in my situation.
After idling in total about 30 minutes, with the belly pan removed, I don't see any leaks ok the ground.

So this is weird.

I'm now thinking back how it all started.
The vehicle overheated, obviously raising the pressure, causing the radiator cap to allow the pressure to escape, and causing the lower radiator hose clamp to come off.
This started the quest for a leak, which i found on the lower radiator hose and fixed.
Hard to say if the leak was caused by the overheating, or if it was there before.
But definitely I had a small leak for the past few months, and I've been seeing the message of low coolant, but vehicle never overheated. I even took it on trip of 500km.

So maybe I had a small leak at the radiator hose, which became a big leak when it overheated.

But now it seems I'm not loosing any coolant, at least when I'm idling. Or maybe I'm loosing coolant, but it's not leaking on the ground. Just idling at 95C, there's nonsmoke coming from X5.
I start driving, within 3 minutes or so, my temperature is up to 115C from 95C

So the mystery is why when rpm is higher, the vehicle overheats. This is pointing me in only one direction, broken water pump. I don't see any other explanation for this behavior.
Or maybe the thermostat only opens partially, like 10% or smth.
I'm inclined to replace both the thermostat and the water pump.

New ≠ fixed you have to follow the symptom which is pump or something blocking the flow.

At idle the car is pushing 5-6hp and driving prob closer to 15-20 steady state to 225 full throttle. A lot more heat to dispose of.

If the temp is stable at idle there is flow so tstat not completely blocked and opens when it shoild if temp is stable.

I haven't seen example thread of the metal impeller coming loose from the shaft but it exactly matches your symptoms.

I think you can pull off the hose and reach in with a tool or finger hold the impeller and turn the pulley to confirm impeller is loose on the shaft or not.