My stupid self forgot to put the cap back on, drove 40 miles, and finally spewed coolant everywhere. My temp solution is duct tape and a pvc cap. If I keep temps under boiling, I should be fine. Will this solution work if my car decides to get up there in temps (sitting in traffic )? My only concern is there isn't any type of breather. Car never overheated btw.

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The cap is a pressure cap...I think I saw a picture of a car with a regular auto parts store "stant"(I think) brand one....if you get the right PSI for the relief, it should be fine. Not sure what the PSI is without looking...
Pressure is used in the system to increase the boiling temp of the coolant...to prevnt boiling, and bubbles, and cavation around the pump...so running unpressurized should be a temporary solution....
That having been said...just keep the tank full enough to make up for the stem that WILL escape, and do a test when you get the cap to determine how much water/coolant will bring the freeze point to the correct point.
You temp solution is better than having it open..it will prevent sloshing...But I doubt it is pressure tight!
If it is a day or two...just keep the fluid topped off (correctly filled...too much and it will slosh out).

 

Yea I just need to get home, about 30 miles. I think I should be alright. If you get the chance, can you check the psi on the cap? I was assuming an auto parts store wouldn't carry these (you know how BMW likes to make certain things you can only get from the dealer).

 

The picture I saw might have been a car with a replacement tank..not sure. I think 30 miles is fine...heck folks with a cracked tank have done thousands...so if you just check the level, drive sane, and watch the temp, all should be fine. Heck since it is kinda cool this time of year, I would not sweat it waiting a week (if you watch the fluid level) while you wait fir the new cap.
If you still need the psi, I go check it in 20 minutes or so...I'm guessing you already drove, but I'll still get it.
+1 for creative use of duct tape!! Heck it will be good if it is not preasure tight...that way it cannot overpreasure!! I Think I had mine off for a day or two once, no worries!!

 

Sorry, cap/system in unmarked....and after looking at it, I think the aftermarket cap cannot work. I guess you are right.

 

I'm going to give Mini a call tomorrow (if they're open) and see if they have one in stock. If not, I'll order from the closest online vendor. It's not a huge issue since I work from home.

I made it the 30 miles fine. I was watching my Scangauge the whole time. Ambient temp. was around 50f. I saw the engine temps reach operating temp astonishingly quick, but once the t-stat opened, the entire ride home my temps were right around 180 (I have a stock t-stat, usually hover around 193... figure that one out..) Hit 204 while rolling through my neighborhood, but never got to a boiling point. I think I heard it "burp" slightly when I shut off the car, but nothing more. She's just sittin' pretty right now until tomorrow.

What sucks is that I was trying to figure out why my brakes were clunking/helicoptering when the plume of white smoke bellowed out of my engine bay, so that is unresolved and hanging over my shoulders right now...

 

I just noticed this tank is nearly empty on my Cooper.. it was about half full when I bought it four months ago. Until I can get to a shop to have it looked at for leaks, can someone tell me what brand of coolant I am supposed to fill it with? Thanks!

 

50/50 BMW blue/distilled water

 

Great, thanks for the quick reply!

 

Looking closer, it seems like the last owner had some Green Stuff in there. Since the BMW stuff is blue, should I be concerned about mixing? Is it better to top it off with distilled water until I can have it properly flushed?