[tcom]

FAiled head gasket will cause the coolant in the radiator to bubble and foam. It will also pressurize the radiator but unless the cap is near new the cap will allow coolant to exit through the overflow tube.

Remove the plugs, if one or two next to each other look like they have been cleaned you have a failed head gasket. THen check compression and if it's low in two adjacent cylinder bores, head gasket is the problem most of the time. cracked or warped head next. Cracked block the least, but it does happen.

ANything over .005" from flat, reface the heads. Don't go cheap on that. Head gaskets on those motors is pretty easy. Clean everything well, Use CopperKote or similar on the gaskets. Chase the threads on the studs and nuts, torque to spec following the pattern in the manual, basically center out in a couple of stages. Retorque the heads after one or two heat cycles. Good to go for another 28 years.

Or you could take it to a shop or a buddy with a test kit and use the blue dye kit where
you can watch the blue turn to yellow