the temp gauge may be the real culprit as well as the level of
antifreeze in the radiator....it might not be as hot as you think it is and perhaps is slightly overfilled and while not puking fromo the cap, the overflow is trying to tell you to let the level go down.
i have a modified K code 289 in my 65 mustang convert, bored .040 over ( 294.3 cubes ), built to shelby gt 350 specs with addition of crane alum roller rockers, 715 lemans holley and tremec 5 spd with 3.89 limited slip rear end . tri-Y headers and aluminum intake and
oil pan. cap is 15# or 16# ( car is offsite now so i cant check it ).
my old stock temp gauge would peg to H but nothing would puke out of the radiator, but overflow would spittle some when stopped . i have the hi-po standard radiator shroud. i added a 6 blade clutch fan, no change in stock temp gauge reading. i finally added an aftermarket temp gauge ( with numbers instead of H or C like std gauge ) and lowered the rad fluid to just above the core in the radiator. anything more and i still get spittle out of the overflow, but if just to the top of the core i get NOTHING. fluid is fresh and 50/50 mixture .
temps run about 190 -195 F in traffic and 180 or so otherwise, unless ambient temps outside are 90 + F . at least now i know what the temps are vs H or C reading guesstimates and confirmed that the old tired stock gauge was innacurate and giving me false readings of being H when it was still in the ok zone in reality.
as i was told by some friends ; if it isnt puking it is not hot ...might be some seat of the pants logic there.
btw , my 57 bird does the same thing re overflow tube spittle if rad is filled above the core. . odd coincidence perhaps, but little birds were hot running engines anyway. old gauge there would peg H prematurely too. I added a 10 ohm resistor and voila, tricked the gauge to showing lower temps , so now if it is really hot it is at H and not giving me an early warning of saying it is H when it really is not.
Bill.