head gasket symptoms?

[67fintail230s]

Hi,
I have a 1975 230.4. Yesterday, on the highway, I noticed the temp gauge was spiked. It happened 25 miles into a 200 mile road trip. The air temperature was a beautiful 85 degrees F. We pulled the car off the highway. The radiator cap relief valve was clicking as the gas evacuated through the overflow. There was no indication of a leak. My first assumption was a failed waterpump which would cause the uncirculated fluid to overheat and boil over. Once the car was cool enough to add water (~3/4 of a gallon), a restart confirmed the water pump was not seized and was operating with no unusual sound.
My new theory is that the head gasket failed between a cylinder and a water jacket. The cylinder pushes air into the system. This causes superheated air pockets. These bubbles expand the fluid and eventually make it to the radiator cap and make the overflow look like a green volcano. I managed to drive the car back home with the temp gauge hovering at about 190 (normally 170) and the heat at full blast.

If any one supports or disagrees with this theory or has any suggestions before I order the gasket kit, I would really appreciate any input.

Thanks

Anthony

[Kestas]

Though I'm not familiar with your engine this failure mode you described is not unusual and very real for a head gasket failure.