Drove about 10kms today, stopped in a carpark for a few mins, went to reverse and steam started coming out of the left-hand side of the bonnet.
Quickly put the car back in the park and turned off the engine, looked outside and there was coolant everywhere (at least a few litres worth). Opened up the bonnet and the cap had popped off!
I don't think the car overheated, I check the temperature gauge regularly while driving after the thermostat went. Although it was a hot day (at least for NZ) and I was driving a little aggressively. If it had popped up quickly I wouldn't have noticed I suppose. I did check the gauge but only 10 mins or so after I turned the car off, and it had fallen down to ~60-70.
I had a look at the coolant that had leaked out, and it was all bright green and clear - couldn't see any signs of oil in there. The coolant expansion tank was empty, but there were a few little flakes of dirt of some kind (most likely insignificant), and there were parts of the plastic which were now black which weren't before (as though the plastic had been burned). The expansion tank + cap is brand new (1 month old).
Pulled out the dipstick and the oil looked clear and oily, no white streaks. Didn't check under the oil cap though, was too hot at the time and didn't have gloves.
The burn marks and coolant bursting the tank open sorta points to a head gasket issue, but I've read that it's pretty rare on the V6 engine and given there's no apparent mix between oil and coolant, I'd like to see if it could be something else.
The mechanic mentioned that my radiator was a little worn out and had a very minor leak in it, but he said to just keep it topped up and not worry about it too much as the leak was so small. He also mentioned that the water rushing sound I get when I start the car could be due to air pockets, potentially caused by a head gasket failure.
I've read a few threads about overheating / coolant trouble on here but couldn't find any threads with the exact same symptoms. But then it's an Alfa I guess, so each car will fail a little differently
Anyone got any thoughts?