I just saw this thread. As soon as the pressure tester went to 23psi there was your answer. How can a 15lb cap seal 23psi? It can't. Thus the coolant fills the overflow bottle. After working in dealerships over 27 years I can tell you head gaskets fail on a regular basis w/o being overheated. All types of engines.
Doug
I don't understand your theory. The pressure tester's cap replaces the vented cap. The pressure tester has no vent to expansion tank. The pressure can rise till the hoses (or something) bursts by either pumping up enough air pressure, or coolant thermal expansion when running engine. BTW, the cap is 21 psi on this engine so 23 is just a tad over.
