[burntoutrubber]

My son had one of my e46s and it had a coolant leak. Took it to the shop and they replaced the radiator and the lower coolant hose. No need for commentary about DIY - I can't do it when I live 1500 miles away.

we drove the next day about 100 miles and the car overheated - steam billowing out of the hood. The bleeder screw was a bit mangled and steam was pouring out.

my question is what would cause this? Presuming I didn't destroy the engine, I would like to repair it, so I will replace the upper radiator hose.

could it have been overfilling or bad bleeding?

 

[burntoutrubber]

I didn't do the repair, some shop did. Red light did not come on, so I am going to replace the upper radiator hose and use your bleeding method.

 

[burntoutrubber]

Done. Pray it works!

 

[burntoutrubber]

It was at 80mph on open freeway.

I suspect either the bleed screw wasn't fully seated or it was cross-threaded. There was a lot of melted plastic around the threaded area of the hose fitting suggesting hot coolant coming out for a significant period before it hit a tipping point.

Put a new upper radiator hose in and have driven it 200 miles with OBDFusion on and it is cooling perfectly.

 

[burntoutrubber]

I replaced the entire upper hose. The threaded opening was pretty melted by the escaping steam. Works perfectly now.

 

[burntoutrubber]

I love to see a pic of the melted thing.
Don’t think it damaged by hot steam; human did that trying to fix the leaking after realizing it was cross threaded.

It was 100% damaged by the heat - it had plastic filaments around the opening. That is not to say that the tech didn't introduce the leak, either by cross-threading or not tightening, but the steam definitely compromised the plastic:

 

[burntoutrubber]

nice to hear 👍🏻
if you did it yourself make sure the connections are tight between hoses, use only bmw hoses for best fit, bleed properly and keep a good eye on the temp for a few drive cycles.

Yep. Definitely not my first BMW cooling system rodeo
It was fun doing the fix in my in-laws' driveway, however.

 

[burntoutrubber]

Nah, you can see the plastic filaments. Trust me, there was heat damage to the plastic. Remember that the high pressure would increase the local temp if the leak were a small opening.

 

[burntoutrubber]

This engine would be damaged before its steam can melt the any parts of the hose. It’s none sense.

Guys, I truly appreciate the analysis, but as Thoreau once said, "Some evidence is incontrovertible, like finding a trout in one's milk." Whether any of us believe it could or could not happen is not as material as the fact that it did happen. The top of the bleeder screw opening was degraded and there was a mixture of granular and filament plastic adhering around the opening. It was not stripped threads, in fact it looked like a bad 3d print run with a clogged nozzle.

The engine runs great now with no compression loss. My only explanation is that the flow at high pressure through a small opening created a local rise in heat at that point.

Look at the area with the circle. The little "ramp"on the upper right side of the bleeder opening is melted plastic. There is no other source of heat than the escaping gasses, and the shape of the deformation is consistent with the escape of hot gasses at that point.

 

[burntoutrubber]

See it to believe it.
I don't understand why the hot steam coolant that melted the bleeder but didn't melt the same plastic on the tank connection port in the same pic. Why? They should be at the same temperature.

That is a good question, but I really believe it was the escaping gas that created a point of high heat that was different from the circulating gasses. I say it because the melted plastic forms a flare that matches in shape what one might expect of escaping gas. I can say that when I first opened the hood that the overheated plastic felt precisely like recently extruded plastic in a 3D printer.