Hi - we've got a Fiat barchetta that is over heating, It boils the water in the expansion tank and once driven then sat in traffic the temp gauge shoots up! i've had this problem on a golf i had years ago but can't for the life of me remember what it was or how i fixed it!

I thought it might be the Headgasket so put my compression tester on and got 105psi, 95psi, 105psi, 120psi. there doesn't seem to be any oil in the water or gunk in the oil cap but i stopped running it to avert anymore damage being done.

Looking at the pressures would you say that the Headgasket has failed or could it be something else such as thermostat?

P.s before anyone says it, i'm not gonna punt it on as it has sentimental value - just wish my dad had bought a vx220 or even a mx5 instead!!!

 

Could be a number of things, cheapest of which is a duff tank cap, ie if not sealing well it lowers the boiling pint enough to cause this, a pump not circulating, how is the heater ?, how long does it take to boil the water, thermostat.
You can have perfect compression even with a blown HG, as it depends where it has blown.
I would start with a new stat, after checking the cap and that water is circulating.
tabetha