Well, first, therms themselves fail so don't think there's a method the car uses to cool itself off when it otherwise can't. Failed therm, wp are probably the leading causes of overheating...and of course, cracked ET.
If it was me, I'd fill up with just water and run it after bleeding and see if you fixed the issue. If you did, drain completely and fill with 50/50...just saves a little trouble and expense in case you have to do something over.
Anything can leak, and the cap itself does above 2 bar, but that's a safety, not a control.
Also, overfilling might lead to coolant leaving from cap too...you need that buffer of air in top of ET...don't overfill...I'm sure you know! Fill to between the two balls on the stick and you'll be safe.
Also, since this sounds like your first cooling issue, be sure to study bleeding threads...you know, until you're sticking needles in your fingers hoping it will lessen the agony...it won't!
Oh, btw, if you have a manual and only one fan, that issue you mention might well be the reason for your overheating...though the therm and wp are always welcomed additions to our cars!
Last edited by dmax; 06-16-2011 at 06:41 PM.