Thread: air/combustion gases in cooling system

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Hello having issues with recently purchased blown 427 in 32 coupe, Initially car only had 16" electric fan, knew this would not work ordered new fan with custom shroud, temp gauge would not work, figured out had air around temp probe would bleed air off and would work a short time then stop again with air around it,did block leak test and indicated minor amount of combustion gas in coolant, thought head gasket , tore down nothing obvious wrong with gaskets, both #5 and # 4 piston tops were very clean, reassembled, doing same thing, I have a set of 1971 LS6 heads when looking into both #5 and #4 exhaust ports i see there is a passage that goes up to where intake gasket mates to heads I am thinking that these passages and clean piston tops are definately related but not sure where problem could be, engine only has 230 miles on build,also both spark plugs in those cylinders appeared sooty, have recently did radiator pressure check and does bleed off from 17 lbs to nothing in about an hour no visual coolant leaks, bought scope to view in cylinders cant see anything leaking but really cant view entire combustion chamber any help would be appreciated only put 35 miles on car since buying in June really frustrated thanks

Common that the one exhaust seat at the end of the head will often go thru to coolant-------experience can be $$$$$$$$$ly.
also if this is a intake manifold with exhaust passage under carb it will connect toport opsite side-higher blown pressure can feed across engine on overlap between those cylinders.

add some block sealer to cooling system and follow directions-------

If the heads are still off have them disassembled and pressure tested.