dd131;

...an interesting point, but I don't believe there is a contradiction here...I believe the fact that you had a sealed cooling sys explains why you did not experience problems even though you had a "bypassless" t-stat...as the coolant of those systems expands into the expansion tank, it tends to burp out any air in the process (drawing coolant back in when it contracts)...so that eventually there is NO air (or very little) in the cooling system at all (and t-stat is obviously fully surrounded at that point), which supports the original theory.

I think this is a subtle but significant advantage of the sealed systems...and like you, I've never experienced a problem with a sealed cooling sys either...so I always upgrade to sealed system and recommend it...I also like the fact that there is NO coolant loss...

Cheers