I'm still having a slightly worrying problem with cooling on this 16V.
Short history is thermostat replaced with a genuine Fiat part, coolant replaced, system bled - I think. HG job done about 18 months ago. Car usually drives with needle just under half way. However it can then drop down to quarter mark and back to half way for no obvious reason in just a few seconds. Sometimes when idling the needle's at halfway and there's not much heat from the car heater but when the engine is reved up then there's heat. I haven't had the car actually overheat yet.
Noticed some coolant the other evening which had been blown out past the filler cap. I bled the system again. No mayo on the oil cap, nothing in the coolant. Sometimes when I've been monitoring the coolant over a few days the level seems to be higher in the overflow tank than in the previous days.
Is it possible the HG is starting to fail again and pressurize the system? Could it just be the filler cap losing spring pressure? Temp sensor?
I'm out of ideas.

Sounds like a massive air lock

The heater matrix is full of air = cold air
Rev the engine, the coolant fills the heater matrix = Some heat

The Coolant level boucing all over the place = Air

The needle going hot cold hot cold is not uncommon of trapped air

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i'll say this, some punto's are pure PIGS to bleed, as they take several attempts and lots of coolant loss....

I've been tempted to try and figure a way out to make a recirculate water/air to heater tank, but it'd be more trouble then its worth....

Ziggy

even if coolant is on max you could still have air inside, had that on my mk1 in january this year, antifreeze was on max but no hot air commin through vents, i filled it to top and bled it -air was commin out of bleed screws, later found out why it happened, water pump was leaking due to failed gasket sealant

first look for leaks and if there arent any, add mixture of antifreeze + distilled water and try to bleed it, if it leaks somewhere just add normal water

antifreeze commin out on top could be just bad cap

Thanks for those replies Ziggy and flisko I think you're right about an ongoing air lock or maybe air being continually added somehow. Can I bleed the system in a different way. I've done it so far by topping the bottle up to the very top and then opening each bleed screw one at a time while also squeezing the top radiator hose to force coolant out of the bleed points. All with engine off.

few bleeding techniques, soon in fiat brochure

https://www.fiatforum.com/punto-mk2-2...t-renewal.html
https://www.fiatforum.com/punto-mk2-2...at-change.html
https://www.fiatforum.com/punto-mk1-g...l-engines.html
https://www.fiatforum.com/punto-mk1-g...mk1-punto.html